Deep Web (also called the Deepnet, Invisible Web, or Hidden Web) is World Wide Web content that is not part of the Surface Web, which is indexed by standard search engines. It should not be confused with the dark Internet, the computers that can no longer be reached via the Internet, or with a Darknet distributed filesharing network, which could be classified as a smaller part of the Deep Web. Some prosecutors and government agencies think that the Deep Web is a haven for serious criminality. (1)
References for further information:
1. Wikipedia write-up: http://en.wikipedia.org/wiki/Deep_Web
Search Engines for Exploring the Deep Web:
Infomine
http://infomine.ucr.edu/
The WWW Virtual Library
http://vlib.org/
Intute
http://www.intute.ac.uk/
Complete Planet
http://aip.completeplanet.com/
Infoplease
http://www.infoplease.com/index.html
DeepPeep
www.deeppeep.org
IncyWincy
http://www.incywincy.com/
DeepWebTech
http://www.deepwebtech.com/
Scirus
www.scirus.com
TechXtra
www.techxtra.ac.uk/index.html