Visual depiction of the architecture of 3 different decentralized social networking protocols

Fediverse

First of all we have Mastodon (I often call it by this name because Mastodon is the de facto controller of the protocol), but a better name would be the Fediverse, which is ugly, but still better than ActivityPub, since "Activity" depicts a much broader protocol, conceived by the minds of unreasonable academics, unimplementable in nature, which was only partly adopted by Mastodon.

Anyway, it is very straightforward: clients talk to a single server, that server talks to other servers.

Nostr

Well, Nostr has been described elsewhere many other times, but its core new idea is that clients can talk to multiple servers, which gives us a very chaotic ecosystem of signed messages.

ATProto

Finally we have Bluesky, or, if we go by the name of the overall protocol, ATProto. Its core new idea is that the basic functions of a social networking server can be broken up in mainly 3 different kinds of servers, which then talk to each other in a pipeline.