This article by rw is a good non-technical introduction to GNU.social, Mastodon, and the protocols and ideologies behind it:
The protocol OStatus is shared by a number of implementations, which are all more or less interoperable. One of the implementations is GNU.social, another is the right now hyped Mastodon. Each of the implementations has many instances, some of them large, many of them very small. They all connect to each other and talk to each other, through federation, and together they form the so called Fediverse.
You can subscribe to one or more of the instances, or start to run your own – it’s up to you.
I signed up as Isotopp@octodon.social, and use it mostly for reading. I won’t start posting there any time soon. So, what is it like?