What if a search engine searches other search engines?

Most search features use the results of others verbatim. Only four actually search the web and are therefore engines: google.com, Yahoo.com, Bing.com and Ask.com. The rest just enlist the help of those four, and this is normally indicated by a "Powered by ..." notice at the bottom of the page.

An engine which combines them ("Powered by google, Yahoo, Bing, and Ask.com") is known as a meta-search engine. A list and more detailed description of these can be found at the related Wikipedia links.