Activitypub is the standard all the fediverse application use, and that's what makes the various apps cross compatible (you can reply to a Lemmy thread from another Lemmy instance, or from a kbin one or even from Mastodon).

This standard is used by various WebApps to provide social networks experiences akin to the big centralized networks.

Lemmy and kbin work like reddit.

Mastodon and pleroma do microblogging (default message length is 500 char, can be modified on each server though).

PeerTube is a distributed YouTube alternative, again using activitypub to federate.

Now thread is a new microblogging app by Instagram and it happens to use activitypub, which means it could be interconnected with fediverse apps. You could in theory follow and interact with its users from any of the fediverse apps. But this feature is not on from their side at the moment. And many fediverse server admins plan to block federation with threads.net because they don't trust Instagram / meta.