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]
In-depth guide to how Google Search works by Google. Crawling, Indexing and Serving search results
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.
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