A web search engine is designed to search for information on the World Wide Web (WWW). (By the way, there are other networks operating on the Internet).  The search results are usually presented in a list of results and are commonly called hits. The information may consist of web pages, images, information and other types of files. Some search engines also mine data available in databases or open directories. Unlike Web directories (Yahoo.com) , which are maintained by human editors, search engines operate algorithmically or are a mixture of algorithmic and human input.

In fact there are two technologies the search engines operate:

1. Information seekers could browse the directory which is prepared by humans. 
2. Keyword-based search. This requires keeping of full-text copies of web pages on the providers' servers. This form relies much more heavily on the computer itself to do the bulk of the work.


When a user enters a query into a search engine relevant field (typically by using key words), the engine examines its index (result of analytical computer-work upon full-text documents collected by search engine Web crawler) and provides a listing of best-matching web pages according to its criteria, usually with a short summary containing the document's title and sometimes parts of the text.

It is easy to understand, that you will get desirable results depending on two factors:

1. Effectiveness of algorithms the programmers of search engine company use to satisfy users' request. 
2. Skills, knowledge and sharpness users compile the "search query" and change it to reach desirable result. It is not so easy as some people used to think and so called boolean operators AND, OR and NOT to further specify the search query. 
Anyway, there are three leader in search engine business - Google, Yahoo! Search, Bing and Baidu. More statistics is here (does not work in Opera)