Search engines work as follows. This information is then used to create an index of web pages which you can search by typing in keywords. When you use a search engine you are not searching the web, you are just searching an index to part of the web. Links to some of the more useful services are in the Searching section of the Academic Gateway . Search engines have many limitations.
None has comprehensive coverage of the Web.
They index every page they find but no attempt is made to assess the quality of the pages indexed. This means that although you may get a great many references from a search, you often have no way of identifying quality resources among trivial or unreliable ones. Search engines can usually find individual pages which mention your topic but they can frequently miss whole sites which cover it extensively. For many searches it is better to use search tools which rely on human compilers, such as subject indexes and information gateways . Subject Indexes .