They are designed to scan the World Wide Web to search for details of sites and pages. This data is then stored in searchable indexes or databases. You can search the contents of these databases by typing selected keywords in the text box provided on the search engine's home page. Basically search engines are made up of three parts. The spider will return to sites periodically to examine their pages for changes.
Information located by the spider is stored in the second part of the search engine, it's index.
It is important to note that when you search using a search engine, you are searching only the contents of that search engine's index, not the contents of the entire World Wide Web. Input your terms once and the program searches across several search engines in a single operation. Duplicate matches are eliminated and the results from each individual search engine are combined into the one result list. The main advantage of using a metasearch engine is that it gives comprehensive results. The programs draw on the resources of several search engines at the same time and involve only one step.