A program that searches documents for specified keywords and returns a list of the documents where the keywords were found. Although searchengine is really a general class of programs, the term is often used to specifically describe systems used by Google™, MSN Live™ and others.

Typically, a search engine works by sending out a spider or crawler to retreive 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 it's own algorithm to create databases that, ideally, return only results relevant to the search query.