The visible Web, or the Web as it is more commonly known, is comprised of HTML documents. They are crawled and indexed by search engines. Taken together, the information they contain is sometimes called the surface Web.
The invisible Web is the information that is hidden below the surface and runs very deep, hence it is also called the hidden Web or the deep Web. A recent study estimates that the invisible Web is 500 times larger than the visible web. The invisible Web is comprised mostly of database content that can only be found using the search interface provided by the site the database is connected to, as well as sites that require registration or otherwise limit access to their pages.
As a large amount of useful data and information resides in the invisible web, search engines have begun exploring methods to crawl the web in order to capture such information.