A program that searches documents for specified keywords and returns a list of the documents where the keywords were found. Although search engine is really a general class of programs, the term is often used to specifically describe systems like Google, Yahoo, Bing, Alta Vista and Excite that enable users to search for documents on the World Wide Web and USENET newsgroups.

Typically, a search engine works by sending out a spider to fetch as many documents as possible using search keywords/keyphrases. 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.

Types of Search Engine:

There are basically three types of search engines;
1) Crawler Based Search Engines
2) Hybrid Search Engines
3) Human Powered Directories

Key Terms To Understanding Web Search Engines:

spider trap
A condition of dynamic Web sites in which a search engine’s spider becomes trapped in an endless loop of code.

search engine
A program that searches documents for specified keywords and returns a list of the documents where the keywords were found.

meta tag
A special HTML tag that provides information about a Web page. (Later we will see what are all the Meta tags would help to improve good results in search engines.)

deep link
A hyperlink either on a Web page or in the results of a search engine query to a page on a Web site other than the site’s home page.

robot
A program that runs automatically without human intervention.

Note: I believe this information would help you to understand about search engines and it types.