In the context of the Internet, search engine usually refers to the WWW and not a protocol or other areas. In addition, search engines collect data available in newsgroups, large databases, or open directories like DMOZ.org. Because data collection is done automatically, unlike search engine web directory of human work.
Most search engines run by private companies that use the algorithm of ownership and private databases - the most popular is Google (MSN Search and Yahoo! Lagging slightly behind). There have been several attempts to create search engines with open-source (open source), for example, is Htdig, Nutch, and OpenFTS Egothor.
source : http://id.wikipedia.org/wiki/Mesin_pencari