There is no controlled vocabulary or list of standardized terms or descriptors for the Web. Because there is no controlled vocabulary, the use of synonyms and variations of keywords to describe your search query is very important. The operation of each search engine varies. Most use automatic software programs known as spiders, worms, robots and gatherers, which roam the Web and collect URLs or addresses of Web sites to add to their database. Some examine a Web page and follow each link to other links.
Most search engines are free.
They obtain their revenue from advertisers who pay to have their ads or services placed on the home page of the Web search engine.