The deep web (also referred to as the invisible Web, hidden Web, or deepnet) is a large section of content on the World Wide Web that isn't catalogued by standard search engines. Most of the deep web contains harmless information, but it can also foster serious criminal activity. The deep web is quite large; several times the size of the surface Web.
The phrase "deep web" was coined by Mike Bergman, founder of Bright Planet. Bergman has made Internet searches analogous to running a fishing net across the surface of the ocean. A lot may be caught in the net, but there is a vast amount of information farther down that the net doesn't get caught. Web pages can end up in the deep Web if there are no other pages linking to them or because they use dynamic databases.