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A web crawler automatically discovers and fetches web resources, often to supply a web search engine. Starting from known addresses, it can extract links and schedule newly discovered addresses for later fetching. Discovery, fetching and indexing are distinct: a fetched page need not be accepted into the searchable collection.^[ref]

In a toy web, page A links to B and C, while B links back to A. Recording previously encountered addresses prevents that cycle from causing endless rediscovery. A pending-work list determines what to fetch next; revisits are still useful when pages change. Real web crawlers also control request rates and handle unavailable resources. This makes crawling a continuing resource-allocation task rather than a single traversal that permanently captures the entire web.

A web search engine performs information retrieval over material gathered from the web. A web crawler discovers or revisits pages, indexing organizes their contents, and query processing selects and ranks results. These are separate stages: a search request ordinarily consults a prepared collection rather than downloading every page afresh.^[ref]

Suppose a newly created page links to a research report. A crawler can discover the report through that link, but discovery does not guarantee successful fetching or inclusion in the searchable index. Even an indexed report may not rank highly for every query. This separation explains why a result list is neither a complete inventory of web content nor a direct measurement of scientific correctness. Link analysis, including the HITS algorithm, can supply ranking evidence alongside textual matching.