So, you probably think that you are pretty familiar with the Internet. Well, the pages you surf through Google etc. are but a tiny part of the vast Internet universe. The Deep Web, it is estimated comprises about 96% of the Internet and the other 4% is those sites which are visible to the public. The Deep Web, as it is often called, is not indexed by standard search engines; it is the Wild West of the Internet universe and you may not want to go there.
The Deep Web is the cyber world where paedophiles dwell. But then there are the arms dealers, drug pushers, illegal passports for sale, terrorists and hit-men for hire. Buyers use BitCoin, a digital currency when paying for their purchases, or services. There is hardly anything you cannot buy with BitCoins. To access the Deep Web you need a special kind of software called Tor (originally short for The Onion Router) which enables users to communicate anonymously on the Internet by encrypting data through various layers like an onion.
This part of the web, also, has private password only sites, unlinked pages which cannot be web crawled, scripted pages, and file formats not handled by search engines. There is also, lots and lots of educational information.
The horrible nightmare inducing parts of the Dark Web, usually, need to be accessed by links that are found on message boards. But I wouldn't advise you to follow these trails, for the same reason that you wouldn't walk down a dark alleyway, in a rough part of town, in the deep of the night.
The Dark Net: Inside the Digital Underworld