When people want to hide their identity, share, or get their hands on information, which would otherwise be illegal or classified, they head on over to the deep/dark web.
Traditionally what is referred to as the dark web is simply considered as that part of the internet that is has not been indexed by search engines, and it accounts for approximately 90% of the actual information on the Web, we can only access about 4/5% of the actual Web information.
So every corporate intranet is part of the “dark web”, Facebook too since its content can’t be searched by Google. The ‘dark web’ is a bad place though with numerous sites offering all sorts of illegal stuff, like drugs, guns, child pornography and the like.
Dynamic websites too (unless they have been optimized for search engine accessibility like online shops)
Now the dark web is of course also part of the deep web but goes a little further:
While it is possible to access most sites of the deep web given the correct URL this is usually not possible for the dark web.
It includes such things as .onion sites, (websites in the deep web don’t have the traditional .com address) and the equivalents from I2P and other identity hiding software.
Without installing such software first it is not possible to access these sites.
There are quite a few who are dedicated to crime in all its forms (mostly just drug dealing though) but the main intent behind the development of such solutions was to give people critical of their government a way to speak out without having to fear getting found out and killed. This is why a lot of the founding behind the development of TOR comes from the USA. (that does not stop other parts of the US government from trying to outlaw and shutdown this service though, a classical case of left hand vs. right hand).