Search engines provide an interface to a group of items that enables users to specify criteria about an item of interest and have the engine find the matching items. The criteria are referred to as a search query. In the case of text search engines, the search query is typically expressed as a set of words that identify the desired concept that one or more documents may contain.
There are several styles of search query syntax that vary in strictness. Where as some text search engines require users to enter two or three words separated by white space, other search engines may enable users to specify entire documents, pictures, sounds, and various forms of natural language. Some search engines apply improvements to search queries to increase the likelihood of providing a quality set of items through a process known as query expansion.
Index-based search engine
The list of items that meet the criteria specified by the query is typically sorted, or ranked, in some regard so as to place the most relevant items first. Ranking items by relevance (from highest to lowest) reduces the time required to find the desired information. Probabilistic search engines rank items based on measures of similarity and sometimes popularity or authority. Boolean search engines typically only return items which match exactly without regard to order.
To provide a set of matching items quickly, a search engine will typically collect metadata about the group of items under consideration beforehand through a process referred to as indexing. The index typically requires a smaller amount of computer storage, and provides a basis for the search engine to calculate item relevance. The search engine may store of copy of each item in a cache so that users can see the state of the item at the time it was indexed or for archive purposes or to make repetitive processes work more efficiently and quickly.
Notably, some search engines do not store an index. Crawler, or spider type search engines may collect and assess items at the time of the search query. Meta search engines simply reuse the index or results of one or more other search engines.
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