Search engines are programs that search documents for specified keywords and returns a list of the documents where the keywords were found. A search engine is really a general class of programs, however, the period is often used to specifically describe systems enjoy Google, Bing and Yahoo! Search that enable users to search for documents on the World Wide Web.
Web Search Engines

Typically, Web search engines labor by sending out a spider to fetch as many documents as practicable. Another program, called an indexer, then reads these documents and creates an index based upon the words contained in each document. Each search engine uses a proprietary algorithm to make its indices such that, ideally, only meaningful results are returned for each inquire.