There are three darknets to my knowledge. They are I2P, Freenet, and Tor project. This topic discusses these darknets in graphic technical detail on how to access them and the purpose of each individual project.
Freenet is a censorship resistance peer to peer resistance darknet, which isn't very useful in my opinion. Information is hard to search for and is distributed so widely that it takes an excessive amount of time to access useful stuff in a timely manner.
I2P is a more useful very encrypted garlic routing darknet, but it is very slow because of the layers of interconnected peer encryption necessary to maintain anonymous connections, and it isn't very anonymous because your IP address is published during the registration process on that darknet.
Tor is an onion routing anonymous darknet that enables users to access the internet from different points around the internet called exit points, by tunnelling through these exit points and routing much like peeling onion layers. Each exit point has an IP address that isn't your own and allows publishing without revealing the ISP or your details.
The Darknets
Moderator: embleton
Re: The Darknets
There is also GNUNet, a peer to peer darknet that I haven't tried yet.