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A web search engine is software that helps you find relevant information on the Web. It searches an organized collection of information about online resources, then selects and orders results in response to your query. Many search engines build that collection by crawling and indexing web pages.
What is a web search engine?
A web search engine is an information-retrieval system that accepts a query and helps locate relevant web pages or other online resources. The Stanford Encyclopedia of Philosophy describes search engines more broadly as systems for keyword searches of distributed digital text; here, “web search engine” means a service focused on resources available on the Web. Stanford Encyclopedia of Philosophy
A search engine is not simply a directory of every page on the internet. It typically searches an organized index of information about pages, rather than checking the entire live Web each time you enter a query.
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How does a search engine work?
Google Search Central describes three stages in Google Search: crawling, indexing, and serving results. Other engines may use different systems, but these stages explain the basic process. Google Search Central
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1. Crawling discovers and fetches pages
Automated programs called crawlers, bots, or spiders find web pages and fetch their contents. They may discover pages by following links from pages already known to the search engine. MDN summarizes this as a crawler navigating links to find pages. MDN Web Docs
2. Indexing analyzes and organizes information
The search engine analyzes information about pages and may store it in an index so it can be retrieved later. That information can include words and other signals from the page. Indexing is distinct from crawling: a crawler can fetch a page without the search engine adding it to its index.
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3. Serving selects and orders results
When you submit a query, the engine searches its index for resources it considers relevant and presents them in an order. This stage is often called serving search results. Discovering a page does not mean it will appear for every query—or appear at all.
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No. A search engine works from the resources it has discovered and included in its index, and its coverage is not necessarily complete. Google states that it does not guarantee it will crawl, index, or serve a page, even if the page follows its Search Essentials. A page may therefore be undiscovered, crawled but not indexed, or indexed but not selected for a particular query. Google Search Central
This is why finding a page on the Web does not guarantee that a search engine will show it. Search results are drawn from what the system can access and has chosen to index, then selected for the query.
What are vertical and metasearch engines?
“Search engine” can describe systems with different scopes and ways of gathering results. Two useful distinctions are whether a service limits the subject or format it covers, and whether it uses its own index or combines results from other engines. Stanford Encyclopedia of Philosophy
| Type | Scope or result source |
|---|---|
| General-purpose web search engine | Searches broadly across web resources using its own service’s index. |
| Vertical search engine | Limits its search to a topic, medium, region, language, or another defined area. |
| Metasearch engine | Draws results from multiple search engines and combines or reranks them. |
A vertical engine’s narrower scope can support deeper coverage in its chosen area. A metasearch service, by contrast, depends on results gathered from other search engines rather than relying only on an index of its own.
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A search engine helps locate web resources by processing a query. A browser is the software you use to open and view web pages. You can use a search engine within a browser, but they are different tools: one finds and organizes results, while the other displays web content.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why might a page not appear in search results?
A page can be absent for different reasons at different stages of the process:
- Not discovered: the crawler has not found or fetched the page.
- Not indexed: the engine fetched the page but did not add it to its index.
- Not selected for the query: the page is indexed, but the engine does not choose it for that search.
These are separate outcomes; a page’s absence from a result list does not by itself show which stage is responsible. Google’s documentation makes clear that crawling, indexing, and serving are not guaranteed. Google Search Central
Where to learn more about indexes and crawling
Readers who want a technical treatment can explore Stanford University’s textbook resource, Introduction to Information Retrieval, particularly its overview of web crawling and indexes. The resource is dated April 7, 2009. Stanford University: Overview: Web crawling and indexes
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