It does look like Twitter, may a bit function like Twitter... but it's not Twitter. Yes, confusing, isn't it?

First of all, the most important thing is that it is decentralized. What does it mean? It means every instance (server) is a separate so-to-say kingdom with its own set of rules and some unwritten traditions. Unlike Twitter which is centralized and has a universal set of rules and a central command brain. If one "kingdom" falls, it doesn't affect the others and the work of the whole Fediverse will continue as usual, unlike when the "central brain" of Twitter will go down and the whole Twitter is down.

Secondly, you can register on one server, look around, but find it unpleasant. You can move to another - there is an option for that. It allows you to (mostly without loss) retain your followers and follows.

Thirdly, there are different platforms to choose from. Mastodon is just one of them.
Pleroma, Misskey and etc.