A web crawler is an automated program that browses the internet to collect information about web pages. This process is known as crawling. The term is often used interchangeably with crawler, spider, search engine bot, or robot. Major search engines like Google, Bing, and Yahoo use web crawlers to keep their indexes up to date.
Crawlers typically start with a list of known URLs. From there, they follow the links on those pages to discover new content. The information they gather helps search engines understand what a page is about and whether it’s relevant to certain search queries. Without crawlers, search engines wouldn’t even know which pages exist, let alone which ones to show in the results.
Web crawlers aren’t used by search engines alone. SEO tools use crawlers to analyze websites, AI web crawlers gather structured data to train models, and commercial crawlers scrape the web for things like pricing data or news updates.
A web crawler is designed to automatically visit websites, analyze their content, and then navigate to other pages through hyperlinks. This process runs without human intervention and can be performed on a large scale.
A crawler typically performs the following tasks:
The crawler begins with a so-called seed list, a collection of known or specified starting pages. These can be popular websites or manually added pages.
The crawler visits each page and scans the source code. It analyzes elements such as:
Text content
Meta information (like title and meta description)
Headings (H1, H2, H3)
Internal and external links
Images and alt text
The crawler identifies hyperlinks on each page and adds them to a queue (crawl queue). It then repeats the process with each newly discovered link.
The gathered information is stored in a database for later use. Search engines use this data to build their search index, while other crawlers may store it for data analysis or AI training purposes.
These terms are often used interchangeably:
Crawler refers to the act of scanning and navigating websites.
Spider is an alternative term referring to the way the bot “crawls” the web like a spider follows a web of links.
Bot is a broader term for any automated script or agent, including crawlers.
While there may be slight technical differences depending on context, they usually refer to the same concept in practice.
A web crawler acts as an automated visitor to websites. While the core idea is to follow links and scan pages, there’s more behind the scenes. Crawlers need to work efficiently, making decisions based on time, bandwidth, and priority. This is where crawling policies and smart strategies come into play.
Start with seed URLs
The crawler begins with a list of known or manually provided URLs.
Load and parse pages
Each URL is visited. The HTML content is analyzed, including metadata, body text, and all embedded links.
Discover new links
All links found on a page are added to a crawl queue. The crawler then decides which URLs to visit next based on predefined rules.
Store data
Collected content is stored in a central index or database. This index is later used by search engines, tools, or AI models.
Revisit pages
Pages are crawled again periodically to check for updates. The revisit frequency depends on the importance of the page and how often it changes.
Crawling and indexing are two distinct steps:
| Step | Description |
|---|---|
| Crawling | The process of discovering and fetching web pages via bots. |
| Indexing | The process of storing, understanding, and categorizing content for search results. |
A page can be crawled but not indexed, often due to low value or because it's explicitly blocked with a noindex tag.
Major crawlers like Googlebot operate with a crawl budget, a limit on how many pages they visit on a specific site over time. Factors influencing this include:
The page’s importance
Website speed
Frequency of content updates
Server responsiveness
Large websites must have clean structure and solid technical SEO to ensure effective crawling and indexing.
AI web crawlers represent a new generation of crawlers that use artificial intelligence to analyze and interpret web content more intelligently. Unlike traditional crawlers, which mainly follow structured patterns and extract raw HTML, AI web crawlers aim to understand the context, meaning, and structure of the content they crawl.
An AI-powered crawler leverages technologies like:
Natural Language Processing (NLP) to understand human language
Machine learning to recognize patterns and continuously improve
Computer vision to analyze images and visual elements
Semantic analysis to interpret the intent behind the content
This allows an AI crawler to distinguish between a product review and an FAQ, or even generate summaries of pages automatically.
| Crawler | Description |
|---|---|
| GPTBot | Used by OpenAI to collect public web data for model training. |
| Common Crawl | A non-profit project offering AI-ready datasets of billions of pages. |
| Diffbot | A commercial AI crawler that automatically categorizes and enriches content. |
| PerplexityBot | Designed for contextual web understanding for AI-driven search applications. |
AI web crawlers are used for:
Training large language models
Smart search engines
Automated data extraction
Sentiment and reputation analysis
Market and competitor research
Because they understand what they read, AI crawlers are especially useful in situations where simple keyword matching isn’t enough.
The term spider is a commonly used nickname for a web crawler. It comes from a simple but fitting metaphor: just as a spider weaves a web and explores all of its threads, a crawler follows links across websites to discover new pages.
The internet is often described as a vast network of interconnected pages, hence the name World Wide Web. A spider “crawls” from one link to the next, much like how a real spider moves across the strands of its web. This visual comparison made the term popular among early developers and search engine engineers.
Although the terms spider, bot, and crawler are often used interchangeably, there are subtle differences:
Spider highlights the behavior of navigating through links.
Crawler focuses on the process of retrieving content.
Bot is the broader term for any kind of automated script or program.
In most practical contexts, especially when talking about search engines, the differences are minimal, and the terms mean roughly the same thing.
Web crawlers play a central role in Search Engine Optimization (SEO). Without crawlers, your website simply wouldn’t appear in search engine results. Crawlers ensure your content is discovered, analyzed, and indexed. The better your site is prepared for crawlers, the more likely your pages will perform well in search engines.
Crawlers use hyperlinks to move from one page to another. That’s why a solid internal linking structure is important. Submitting an XML sitemap also helps crawlers understand your website faster and more efficiently.
Key elements for crawlers:
Robots.txt: controls which parts of your site crawlers are allowed to access.
Meta tags: such as noindex or nofollow, influence whether a page gets indexed.
Canonical tags: indicate the original version of a page in case of duplicate content.
Structured data: helps crawlers better understand content (like reviews, FAQs, or recipes).
Make sure your website is technically accessible to crawlers:
Use a clean and logical URL structure
Optimize loading speed
Avoid relying too much on JavaScript for key content
Monitor crawl errors using tools like Google Search Console
A page can only be indexed after a crawler has visited it. Indexing means that the content is stored in the search engine’s database and becomes eligible to appear in search results.
Efficient crawling ≠ high ranking. But without crawling, ranking is not even possible.
Although web crawling and web scraping are often confused, they are two distinct processes with different goals and use cases.
Web crawling is about discovering web pages. Crawlers visit websites, follow links, and collect basic information to understand what pages exist and what they contain. Search engines like Google use crawling to keep their indexes up to date.
Key traits:
Navigates through links
Automated and large-scale
Focused on page discovery and indexing
Typically respects robots.txt and crawl policies
Web scraping goes beyond discovery. It's focused on extracting specific data from web pages. Examples include gathering product prices, reviews, contact details, or other content from the HTML structure.
Key traits:
Targeted content extraction
Often used for data analysis or automation
May violate website terms of use
Doesn’t always respect robots.txt
| Feature | Web crawling | Web scraping |
|---|---|---|
| Purpose | Discovering and indexing pages | Extracting specific data from content |
| Commonly used by | Search engines, AI bots | Marketers, analysts, competitors |
| Scale | Large-scale, general | Targeted, often smaller in scope |
| Legal aspect | Generally legal | Legally grey or prohibited, case-dependent |
There are several types of web crawlers, each with its own purpose and functionality. Some are broad and scan the entire web, while others are more focused on specific content or use cases.
These are the most well-known crawlers. They’re used by search engines like Google, Bing, and Yandex to explore the internet and index web pages.
Examples include:
Googlebot (Google)
Bingbot (Microsoft)
YandexBot (Yandex)
These crawlers leverage artificial intelligence to analyze content more deeply. They're often used for training language models, powering semantic search engines, or collecting structured data.
Examples include:
GPTBot (OpenAI)
Common Crawl
Diffbot
PerplexityBot
Companies use commercial crawlers for specific purposes like price tracking, content monitoring, or SEO analysis. These crawlers are typically part of paid tools or platforms.
Examples include:
AhrefsBot (SEO tool)
SemrushBot (SEO tool)
RogerBot (Moz)
These are freely available crawlers that developers can use, modify, and expand. They’re commonly used in research, education, or internal data collection.
Examples include:
Scrapy (Python framework)
Apache Nutch
Heritrix (used by web archiving initiatives)
Some organizations build custom crawlers tailored to their specific needs, such as powering internal search engines or aggregating proprietary datasets.
While web crawlers are useful, as a website owner you may want to control which bots can access your site. Fortunately, there are several ways to manage, limit, or completely block crawlers.
The robots.txt file is the standard method for giving instructions to crawlers about which parts of your site they are allowed to access. This file is usually located in the root directory of your domain (e.g., example.com/robots.txt).
Examples:
User-agent: *
Disallow: /admin/Or for a specific bot:
User-agent: Googlebot
Disallow: /test-page/Keep in mind: this is a guideline, not an enforcement. Not all bots respect it.
With the <meta name="robots" content="noindex, nofollow"> tag, you can instruct search engines not to index a specific page or follow its links. This tag should be placed in the <head> section of your HTML.
You can block bots showing suspicious behavior at the IP level via your server settings or security software. This is commonly used to protect against aggressive scrapers or spam bots.
If you want to prevent bots from accessing forms or specific routes, you can implement CAPTCHAs or require user login. Most bots cannot get past these types of barriers.
For larger websites, tools and services like Cloudflare Bot Management can help automatically identify legitimate bots and block or limit harmful ones.
There are hundreds of web crawlers active on the internet, but a few stand out due to their scale, purpose, or impact. Below is an overview of the most well-known and influential crawlers.
| Crawler | Owned by | Purpose |
|---|---|---|
| Googlebot | Indexing web pages | |
| Bingbot | Microsoft Bing | Crawling for search results |
| YandexBot | Yandex | Russian search engine |
| Baidu Spider | Baidu | Chinese search engine |
| DuckDuckBot | DuckDuckGo | Privacy-focused search engine |
| Sogou Spider | Sogou | Chinese search engine |
| Crawler | Owned by | Purpose |
|---|---|---|
| AhrefsBot | Ahrefs | Backlink and content analysis |
| SemrushBot | Semrush | SEO and keyword analysis |
| Moz’s RogerBot | Moz | SEO auditing |
| Majestic-12 | Majestic | Link profile analysis |
| Crawler | Owned by | Purpose |
|---|---|---|
| Facebook External Hit | Generating link previews | |
| Twitterbot | X (Twitter) | Fetching metadata for link previews |
| Slackbot | Slack | Link preview rendering in messages |
These crawlers typically respect robots.txt rules and behave politely. You can identify them through your server logs or tools like Google Search Console, Semrush, or Ahrefs.
AI web crawlers differ from traditional bots in that they don’t just collect content, they try to understand it. Using machine learning, natural language processing (NLP), and other techniques, these crawlers can recognize patterns, interpret context, and extract structured information. Below is an overview of the most well-known AI-driven crawlers.
| Crawler | Belongs to | Purpose |
|---|---|---|
| GPTBot | OpenAI | Collects public web text to train language models |
| Common Crawl | Non-profit project | Crawls the web to build open-access datasets |
| Diffbot | Diffbot | Converts webpages into structured data (knowledge graph) |
| PerplexityBot | Perplexity AI | Crawls and analyzes content to generate AI-powered answers |
| AnthropicBot | Anthropic | Crawls data for use in AI systems like Claude |
AI crawlers are commonly used for:
Training large language models (LLMs)
Building knowledge graphs
Powering semantic search engines
Data enrichment and classification
Enabling conversational AI systems
While many AI crawlers respect robots.txt, some are relatively new and may follow different standards. More websites are beginning to explicitly block AI bots using this file due to privacy and copyright concerns.
Example:
User-agent: GPTBot
Disallow: /While web crawlers are useful for search engines, analytics, and AI, they can also bring technical and legal challenges. Not all crawlers behave nicely, and some may even harm your website if left unmanaged.
Each crawler sends requests to your server. One bot may not be an issue, but when multiple bots crawl thousands of pages at the same time, it can slow down or crash your website. This is especially risky for smaller sites without caching or scalable infrastructure.
Crawlers may unintentionally (or intentionally) access sensitive information that wasn't meant to be public. This includes:
Unprotected admin panels
Staging environments not blocked by robots.txt
PDFs or documents containing personal data
Not all bots are friendly. Some are built to:
Copy your product prices
Harvest email addresses (spambots)
Steal your content for duplication
Monitor competitor strategies without permission
These bots often ignore robots.txt and may rotate IPs to avoid detection.
While crawling public web data is usually legal, there are legal grey areas:
Copyrighted content
Website terms of service violations
GDPR/CCPA if personal data is collected
In some cases, courts have ruled against large-scale commercial scraping or unauthorized crawling.
If you misconfigure your robots.txt or use the wrong meta tags, you might accidentally block valuable pages from being indexed by search engines, harming your visibility and rankings.
The deep web refers to parts of the internet that are not accessible to standard web crawlers. This means the content doesn’t appear in search engine results, even though it exists online. Crawlers can only access pages that are directly linked and publicly available, without any forms, logins, or session-based access.
Examples of deep web content include:
Pages behind a login (e.g. email accounts, cloud drives)
Search results that appear after filling in a form
Paywalled or subscription-based content
Internal company portals
Dynamically generated URLs without inbound links
Crawlers rely mostly on following links. They don’t interact with forms, click buttons, or log in. Even more advanced bots with JavaScript support often struggle with:
CAPTCHA-protected content
Temporary session-based URLs
Pages that only appear after user interactions
| Feature | Surface web | Deep web |
|---|---|---|
| Link-accessible | Yes | No |
| Indexed by search engines | Yes | Usually not |
| Example | Blog post, product page | Logged-in dashboard, database query |
The deep web is not the same as the dark web. The deep web simply refers to non-indexed but legitimate content. The dark web, on the other hand, is deliberately hidden, encrypted, and often accessed through anonymity networks like Tor.
Web crawlers are the invisible engine behind search engines, data collection, AI models, and many modern technologies. Without crawlers, search engines wouldn’t be able to deliver up-to-date results, SEO strategies would lose their purpose, and AI would be far less intelligent.
Crawlers ensure that information is discoverable, organized, and usable. By continuously scanning the web, they connect users to the right content, whether that’s an online store, a blog post, or a scientific publication.
However, it’s important to manage crawlers thoughtfully:
Website owners should understand how to guide or restrict bot access.
Crawler users must respect legal and ethical boundaries.
Server administrators need to protect their infrastructure from overload or abuse.
In short, web crawlers make the internet functional and accessible, but they also require smart control and clear limitations.
A web crawler is an automated program that visits websites, follows links, and collects information. This data is then used to index pages for search engines or AI applications.
AI web crawlers use artificial intelligence to not only collect content but also understand it. They recognize context, structure, and meaning, and are often used to train language models or power semantic search engines.
No, web crawlers are generally legal as long as they respect rules like robots.txt and don’t violate copyright laws. However, scraping sensitive or protected content can carry legal risks.
Google uses Googlebot, one of the most well-known web crawlers. This bot continuously scans the web to discover new or updated pages to include in its search index.
An example is when Bingbot or Googlebot visits your website, analyzes the content, and follows links to other pages. The collected information is then stored in the search index of Bing or Google.
