World Wide Web is developing rapidly; there are large numbers of web databases available for users to access. This fast development of the World Wide Web has changed the way in which information is managed and accessed. So the web can be divided into the surface web and the deep web. Surface web refers to the web pages that are static and linked to other pages, while deep web refers to the web pages created dynamically as the result of specific search. This literature paper focuses on querying the deep web.