This is a fairly comprehensive post explaining what "the fediverse" is. It does a good job explaining the grey area between the ActivityPub protocol, the different platforms, and the communities.
Written by @diogo
Welcome to Ten Forward, a Star Trek themed Mastodon instance! For more information on what Ten Forward is in the Star Trek universe, please read this page on Memory-Alpha. The instance is not restricted to Star Trek role-play or characters. More general purpose use is welcome.