A search engine is a program that searches for World Wide Web (or WWW, which is why, when we write a URL to get to a certain website, we sometimes use www.) for documents using certain keywords. It then returns a list of documents in which those keywords are found. Search engines like Google are a general class of programs that use a proprietary search algorithm; they rely on probability, linear algebra, and graph theory to create a list that, ideally, produces the most relevant website results for the user.