The Fediverse is a network of thousands of independent social media servers of many different kinds.
The most popular kind is Mastodon, but it wasn't the first and isn't the only one. Some of the accounts you interact with are on other server types, e.g. @Iancylkowski is on Pixelfed and @theatticdwellers is on PeerTube. There are many Fediverse server types:
Whichever type you're signed up on, they connect together to form a social network called the Fediverse.
I don't recommend Lemmy (this is why: https://mstdn.social/@feditips/106835057054633379).
No, it's not that. It's that the lead developers are the ones who run the problematic servers. Did you actually read the thread I linked to? It explains all this.
@feditips thank you for linking this to me