Search engines are for us Internet users initially Internet pages on which we can enter a term, and then list all those websites on which the entered term usually plays a role. This colloquial definition has a lot to do, which is worth taking a closer look at. Because behind the input and output side of a search engine is far more than one suspects at first glance.
Scientifically defined is an Internet search engine “… a computer system that crawls content from the World Wide Web and makes it searchable via a user interface, the results being listed in a representation organized according to the system’s assumed relevance” .
In general, search engines can be divided into three different types : index-based search engines, catalog search engines and metasearch engines . In addition, search engines can be categorized by their search areas: they search either the entire freely accessible network or only partial areas.
Web crawlers (also called spiders) are programs that automatically scour the World Wide Web and analyze web pages.
The most widely used search engine type offers an index-based search. With the help of crawlers, the search engine automatically reads in a large number of WWW documents, analyzes them algorithmically (ie with the help of a computer program) and then creates a search index, which is contacted in later search queries. The best-known examples of index-based search engines are Google, Bing, Yahoo or Ask. The advantage is the speed with which the respective search result lists are displayed as well as the scope of the index.
A catalog contains search results that have been previously compiled and sorted by people and, if necessary, manually weighted. Normally, behind a catalog is an alphabetical or thematically ordered list. Examples are open directories and child search pages . The advantage of this manual offer is that a person has already made a preselection for the future user.
A metasearch engine does not create its own search index, but uses the database of index-based search engines. The individual search results of the searched index search engines are weighted by the metasearch engine and put together in a new result list. The advantage of a metasearch engine lies in the potentially larger database that results from linking the individual assets. Metasearch engines are little used compared to the index-based search engines.
In addition to the web search engines, which in principle search the entire publicly accessible World Wide Web, there are a variety of other search engines.
This list is not exhaustive, only the most common search engine types are listed.