Search engines are programs that search documents for specific keywords and return a list of the documents where the keywords were found. A search engine is really a general class of programs; however, the term is often used to specifically describe systems like Google, Bing, and Yahoo! Search that enable users to search for documents, articles, web pages, and videos on the World Wide Web.
Web Search Engines
Typically, Web search engines work by sending out a spider to fetch as many documents as possible. Another program, called an indexer, then reads these documents and creates an index based on the words contained in each document. Each search engine uses a proprietary algorithm to create its indices such that, ideally, only meaningful results are returned for each query.
As many website owners rely on search engines to send traffic to their website, an entire industry has grown around the idea of optimizing Web content to improve placement in search engine results. Search engine optimization (SEO) uses keywords as appropriately and specifically as possible to make a web page more prominent.
Common Search Engines and Tools
In addition to Web search engines, other common types of search engines and software include the following:
Desktop Search Tool
A meta search engine queries other search engines and then combines the results that are received from all.
Blog Search Engine
Blog search engines search the blogosphere and only index and provide search results from blogs (Web logs).
Recommended Reading: Webopedia’s How Web Search Engines Work.