How do search engine works

There are many search engines available in net. Google is the best search engine among others. We have been searching the data or files generally through this search engines but do we know how these search engines are extracting the files required? There are three types of approaches to extract a particular file from millions of files. They are Keyword-based search, Querying deep Web sources, Random Surfing. The Keyword-based search method uses keyword indices or manually built directories to find documents with specified keywords or topics such as Google or Yahoo. In Querying deep Web sources the information hides behind searchable database query forms such as Amazon.com book data and realtor.com. In Random surfing it follows like Web linkage pointers. So know the process before using it.