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. It can also switch names within the search engines from previous sites. Whereas 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. Query understanding methods can be used as standardize query language. The list of items that meet the criteria specified by the query is typically sorted, or ranked. Ranking items by relevance reduces the time required to find the desired information. Probabilistic search engines rank items based on measures of similarity and sometimes popularity or authority or use relevance feedback. Boolean search engines typically only return items which match exactly without regard to order, although the term boolean search engine may simply refer to the use of boolean-style syntax in a probabilistic context. To provide a set of matching items that are sorted according to some criteria 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, which is why some search engines only store the indexed information and not the full content of each item, and instead provide a method of navigating to the items in the search engine result page. Alternatively, the search engine may store a 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. Other types of 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, dynamically considering additional items based on the contents of a starting item. Meta search engines store neither an index nor a cache and instead simply reuse the index or results of one or more other search engine to provide an aggregated, final set of results.
