A "search engine" is a software algorithm and system that is designed and programmed to search the Internet and retrieve websites that match the online surfer's search words. Search engines are in the business of sorting web pages in their databases by the keywords contained in a web page or keywords used to describe a web site for submission-based directories.

Every search engine tries to give you the best and most relevant sites you are looking for, based on the search words (keywords) you use. They do this because they know you will only try another search engine if their results are unsatisfactory. In other words they try to make the way they list information relevant to the way search engine users looks for information.

Well known search engines include Yahoo, Google, MSN, Excite, Sina, Baidu, Sohu, Netease, etc. The results obtained from a search on these sites will be shown with the most relevant sites listed on the first few pages, according to each of the individual search engine's unique indexing and ranking algorithms. Each search engine will actually produce very different results because they use very different methodologies. See the major search engines List.