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.