A search engine crawler is automated software that discovers web addresses (URLs) and requests pages and other resources so a search engine can process them. Crawling is the fetching step—not indexing, ranking, or a guarantee that a page will appear in search results. Google calls its fetching program Googlebot, also known as a crawler, robot, bot, or spider. (Google Search Central)
What does a search engine crawler do?
A crawler sends automated requests to web servers to retrieve pages and, where relevant, resources such as images, scripts, and stylesheets. Search engines use the fetched material to learn what a page contains and decide whether it can be processed further.
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A crawler is software, not a person or usually one physical robot. The word “spider” is a metaphor for software that follows paths between pages. Google’s documented process is one example; other search engines may use different systems, schedules, and technical rules.
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- Discover a URL. A search engine may already know the address, find it by following a link from a known page, or learn it from a submitted sitemap.
- Decide whether and when to fetch it. The crawler chooses which sites and URLs to request, how often, and how much to fetch. Google says it tries to avoid overloading sites and can slow down in response to server errors such as HTTP 500 responses. There is no fixed crawl schedule that applies to every site.
- Request and process the resource. The crawler fetches the URL. Google may render pages and run JavaScript using a recent version of Chrome; that is a Google-specific description, not a promise about every search engine. (Google Search Central)
A sitemap can help a search engine discover URLs, but submitting one does not guarantee that those URLs will be crawled or included in results.
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How is crawling different from indexing and ranking?
These are separate stages. Crawling downloads content. Indexing analyzes the fetched content and signals, then stores information the search engine may use. Serving results selects information considered relevant to a user’s query. A page can fail to move forward at any stage, so crawling makes a page available for processing; it does not put the page in the index or guarantee a ranking.
| Term | What it means | What it does not guarantee |
|---|---|---|
| Crawling | A crawler discovers or requests a URL and fetches its content. | That the content will be indexed or shown in results. |
| Indexing | The search engine analyzes content and stores information in its index. | That the page will be selected for a particular search. |
| Serving results | The search engine selects results for a user’s query. | A particular position or continued visibility. |
Google explicitly says it does not guarantee that it will crawl, index, or serve a page, even if the page follows Google Search Essentials. Access problems, server errors, crawler controls, or the search engine’s own processing decisions can prevent a page from progressing. (Google Search Central)
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What is Googlebot?
Googlebot is Google Search’s crawler. Google describes two general search crawler types: Googlebot Smartphone and Googlebot Desktop. They simulate mobile and desktop users, but use the same Googlebot product token in robots.txt; Google says site owners cannot target them separately with robots.txt rules. Google also says most crawl requests for most sites come from its mobile crawler. These details describe Google, not all search engines. (Google Search Central: Googlebot)
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Google’s crawler identity should not be inferred from a user-agent string alone: such strings can be spoofed. For Googlebot verification, Google documents reverse-DNS checks and comparison of source IP addresses with its published crawler IP ranges. (Google Search Central: Overview of Google crawlers)
Google’s March 31, 2026, description of its shared crawling infrastructure says Googlebot fetches up to 2 MB from an individual URL, excluding PDFs, and up to 64 MB for a PDF. The post says the limit includes the HTTP header. These are Google-specific fetch limits, not general limits for search engine crawlers. (Google Search Central, March 31, 2026)
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Does robots.txt keep a page out of search results?
No. A robots.txt rule tells compliant crawlers which paths they may access on the host, protocol, and port where that file is served. It is a crawl-access control, not a security boundary and not a reliable way to remove a URL from search results. Google notes that a blocked URL may still appear in results if it is known from elsewhere. (Google Search Central: Introduction to robots.txt)
If the goal is to keep a page out of Google’s index, Google’s guidance is to allow crawling and use a noindex directive. Blocking the crawler can prevent it from seeing that directive. If content is private, protect it with access controls such as authentication rather than relying on robots.txt. (Google Search Central: Block search indexing with noindex)
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What should a site owner take away?
- Make important pages discoverable through links or a sitemap; neither method guarantees crawling.
- Keep crawling, indexing, and appearing in results distinct when diagnosing visibility.
- Use robots.txt to manage crawler access, not to protect confidential information or reliably deindex a URL.
- Use an allowed-to-crawl
noindexdirective when the aim is to tell Google not to index a page. - Check Googlebot claims using Google’s verification guidance rather than trusting a user-agent label alone.
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