What a web crawler is
A web crawler, also called a spider or bot, is a program that automatically browses the web, moving from page to page by following links. Its main job is to discover pages and read their content so they can be added to an index. Search engines are the best-known users of crawlers: Google's crawler is called Googlebot, and it constantly explores the web so that search results stay current.
How crawling works
A crawler starts with a list of known web addresses, called seeds. It visits each one, reads the page, and extracts every link it finds, adding new addresses to a queue of pages to visit next. By repeating this process endlessly, a crawler can travel across huge portions of the web. As it goes, it stores what it finds so that another system can process and index the content later.
Crawling and indexing
Crawling is only the first step. After a crawler fetches a page, a separate process analyzes the content, works out what the page is about, and stores it in a giant database called an index. When you search, the engine looks through this index rather than the live web, which is why results appear instantly. Pages that are never crawled generally cannot appear in search results at all.
How sites guide crawlers
Website owners can influence how crawlers behave. A file called robots.txt tells crawlers which parts of a site they may or may not visit, and a sitemap lists important pages to help them find everything. Crawlers also have a limited crawl budget for each site, so a fast, well-organized site tends to get more of its pages discovered and refreshed than a slow or tangled one.
Crawlers beyond search
Search engines are not the only ones that crawl. Archive projects crawl pages to preserve them, security companies crawl to find vulnerabilities, and AI companies crawl to gather training data. Not all crawlers are welcome, and poorly behaved ones can overload a server with requests. This is why respecting robots.txt and crawling at a reasonable pace is considered basic good manners for any bot.
The bottom line
A web crawler is a bot that travels the web by following links, fetching pages so they can be indexed and searched. It powers search engines, archives, and more, guided by signals like robots.txt and sitemaps. Understanding crawlers explains how a search engine knows about billions of pages, and why making a site easy to crawl is a cornerstone of being found online.
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See also: What is web scraping? Explained, What is deep linking? Explained.
Sources
Published date reflects the original event date (2026-10-23). This article is original Skillo editorial written from the sources above; facts were verified in September 2026.
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