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The Baltimore Sun, of all places has a nice little summary. Basically, it is an encrypted network (The Onion Network or TOR) where all sorts of seedy things can be found. Lots of illegal stuff too, and plenty of people that are serious criminals. Not particularly easy to navigate compared to the online experience most people are used to. If you really want to poke around, you can download the TOR browser bundle and see for yourself. If you do, I encourage you to read all the cautions and suggestions, and follow them closely. It’s not all bad, but make no mistake, you are going on the wrong side of the internet tracks.
"The Deep 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 Internet, or with a Darknet distributed filesharing network, which could be classified as a smaller part of the Deep Web."
As far as accessing the Deep Web:
"In 2005, Yahoo! made a small part of the deep Web searchable by releasing Yahoo! Subscriptions. This search engine searches through a few subscription-only Web sites. ... DeepPeep, Intute, Deep Web Technologies, and Scirus are a few search engines that have accessed the deep web."