All in all, darknets are a simple concept. They are a closed network which are (typically) invite only and exist to provide some sort of content/service. An example of this is the child pornography ring which used usenet mailing groups and GPG keys to talk about, and distribute CP. Everything was encrypted and only members of the darknet knew how to get the CP. Another example is the more popular stuff such as Tor onion sites. They aren't indexed, often no one even knows they exist.
The basic idea behind a darknet is that everyone is anonymized into a cloud nebulous of data and because the group is kept small and everyone is vetted before being able to join, it makes it very difficult for people to infiltrate the group or take it down.
