What is Web Crawler?
A web crawler, sometimes called a spider or bot, is software that automatically browses the internet and collects information from websites. Its main job is to fetch pages and resources from the web. In practice, a crawler is more than just a fetcher. Googlebot, for example, is composed of three systems: the fetcher, a controller that merges links, and a scheduler that determines what and when to fetch.
A Brief History of Web Crawler
Crawlers first appeared in 1993 with the World Wide Web Wanderer, created at MIT. It was not built for search but to track the growth of the web. The data it collected was later used to build an index, which became an early form of a search engine.
At the beginning, crawlers were simple. They could only collect limited information from a page, such as meta tags. As search needs grew, crawlers were adapted to capture more, including visible text, images, alt tags, PDFs, and other documents. These days, crawlers are not limited to search engines. They also play a key role in SEO tools, research, training LLMs and chatbots.
How Web Crawlers Work
When a web crawler visits a site, it typically starts with a set of known web pages, often called seeds. It requests each page, downloads its content, and then extracts all the hyperlinks it finds. These links are added to a queue of new pages to visit. The crawler then moves through its queue, repeating this process continuously. By following links from one page to another, it can traverse the vast network of websites, discovering new pages along the way.
To operate responsibly, crawlers respect rules set by websites in a file called robots.txt. This file specifies which pages or directories the crawler is allowed or disallowed to visit. Additionally, good crawlers implement throttling or crawl delays, ensuring they don’t overload servers with too many requests at once.
Web crawlers also account for compatibility and accessibility. Some crawlers emulate multiple operating systems and browsers to ensure they capture how pages appear across different platforms.
Web Crawlers & Bots types (2025)
Indexing Crawlers
Search engine crawlers, also known as web spiders or bots, are automated programs that browse the internet to discover and index web pages. This indexing process is crucial for search engines to provide relevant search results to users. Crawlers follow links from one page to another, gathering information about the content and structure of each page, which is then stored in the search engine's index.
Training LLM Crawlers
A crawler for training LLMs, also known as an AI bot or LLM crawler, is a specialized program that automatically retrieves and processes web content to provide data for training or enhancing large language models (LLMs). These crawlers are designed to efficiently gather and format text from websites, often prioritizing content relevant to LLM training. They can also be used to extract specific data for refining LLM outputs or for testing LLM capabilities.
SEO Crawlers
SEO crawlers are automated programs that scan websites, much like search engine crawlers, to identify technical and content-related issues that might impact search engine optimization (SEO). They help website owners and SEO professionals understand how search engines see their site and identify areas for improvement.
Live Content Fetch (User Agents)
Live content fetching by user agent crawlers refers to the process where automated programs, identified by specific user agent strings, access and process web content, often in real-time or near real-time. These crawlers are designed to simulate user interactions or gather data.