The Invisible Web: Dark Data of Cyberspace

"Invisible or Deep Web: What it is, How to find it, and Its inherent ambiguity"
UC Berkeley - Teaching Library Internet Workshops (December 4, 2008)

"What is the 'Invisible Web', a.k.a. the 'Deep Web'?

"The 'visible web' is what you can find using general web search engines. It's also what you see in almost all subject directories. The "invisible web" is what you cannot find using these types of tools...."

This is a pretty good discussion of the "Deep Web," and has some practical advice for how to find and use it. If you are serious about research, this may not be a 'must read,' but it couldn't hurt to look the page over.