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?
It’s better than Twitter, and the raised character limit Mastodon presents (500 characters) helps a lot.
The fact that instances separated structures content – people sharing the same instance with you get special treatment, they form their own stream which you can read. So it matters where you are getting an account. So far, the content the ocotodon-local stream is throwing at me is surprisingly hate-free, coming from Twitter, but is also only very mildly interesting.
The fact that it presents content structurally like Twitter, in individual messages with no threading and very little structure, is appalling and makes conversations still hard, even if the character limit is not getting into the way. A structure and presentation that is more like Google plus or Facebook, a thing that is more conversational, would actually be helpful.
The way content is being presented in the default interface shown by Octodon also does not help – the web interface things that every device is a tablet and wastes a lot of screen space, misformats images and is fast, but just plain ugly.
For me, this puts Mastodon rather far back on the reading list and not at all on the post-to list. Which is rather unfortunate, because the ideas behind it are actually not broken.