About Search Engines
Search engines are handy tools that help you find what you want on the Web.
Each search engine uses software (called spiders or robots) to compile a database of pages found on the publicly accessible Web. When you enter a search, the search engine scans its own database to match your terms against terms in the pages of its database.
So, each search engine searches the part of the Web it has collected--not the whole Web--and each search engine has a somewhat different database.