I've only heard of Mastodon. Is Pleroma a Mastodon server, or its own thing? (Searching for Pleroma does nothing but result in Greek Mythology hits).
Mastodon and Pleroma are both part of the Fediverse [2]. They work with any software that works with OStatus and the newer ActivityPub. This includes things like GNUSocial, etc.
Mastodon doesn't explain any of this well, or at all, to my dismay. (As far as I'm aware, on the joinmastodon page).