* AP is a decentralized protocol, like email or XMPP, that standardizes federated interactions from server-to-server, and interactions from server-to-client. Sharing data between end users depends on one or more servers.

* SSB is distributed protocol, more like Git or BitTorrent. It has a concept of "pubs" which play a supernode role vaguely similar to that of BitTorrent trackers, or maybe a WebRTC server. Sharing data between users is P2P, not mediated by a server, although the pubs help user locate each other.

EDIT: formatting