These tools examine and index pages' content, or, when the entire page cannot be processed, the metadata hidden in the sites' HTML tags. Google boasts the largest database, with 560 million fully indexed and 500 million partially indexed pages. AltaVista , FAST Search , and Northern Light each claim over 300 million. Size isn't everything, however. Many factors contribute to a search engine's usefulness, from the algorithms it uses to process the information its crawlers dig up to its interface.
Search engines are statistical systems.
They return exact and partial matches based on a document's probable relevance as calculated by the engine's search algorithm. This is because the directories are hierarchical lists that categorize the Web like a library's card catalog. They work well for the big picture, but the farther down you drill, the less likely they are to have indexed the page you need. Another relevance test we examined is a site's ability to target the specific home page of a site. Unless you have a unique search term, use as many words as possible to describe your query.