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Search engines use computer programs (web crawlers) to automatically visit and download web pages to their local computer systems. They use other computer programs (indexers) to organize the downloaded copies of web pages into lists (indexes) that contain words, references to the web pages that contain those words, and information about how those words were displayed on those web pages (position, bold/italic/etc and possibly more). When a person (searcher) types a word or phrase (search query) into a search box the search engines use more computer programs (search result serving programs) to access the indexes, generate a list of web pages (search results) and display them to the searcher. The order that the search results appear in is determined by another computer program (ranking algorithm) that analyzes many factors in an attempt to sort the search results from most relevant to least relevant. This involves assigning a numerical value to each result that represents its relevancy (rank).
An intermediate search engine is any search engine that pulls some or all of their results from one or more other search engines (which may themselves be intermediate search engines). They may have their own ranking algorithm and/or their own web crawler but they still couple their own results with those of at least one other search engine.
A meta search engine is a type of intermediate search engine that pulls results from at least two other search engines, combines them and displays them together on the same results page. At their most basic level meta search engines just assimilate the results they take from other search engines but some of the more advanced ones implement their own ranking algorithms in an attempt to improve the relevancy of the results beyond what would be possible for any one of the search engines they pull from.
Niche search engines are also called vertical/topical/speciality search engines. They may be crawler-based or meta search engines. They narrow in on one specific category in an attempt to produce the most relevant search results only within that category. Some try to compete with general search engines by offering advanced features geared toward the category they focus on as well as providing more comprehensive search results.
Directories are categorical hierarchies of web pages. The attempt is not to produce a list of results ordered by how much they relate to a search query but instead to collect as many web pages as possible and list them under the category that best describes their topic. Directories are not search engines but some offer a search feature. Both the web page listings and the categories are indexed and displayed on the search results page. |
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