@cwebber The "Fediverse" is a number of different applications, of which Mastodon is one. They all talk to each other using a protocol called ActivityPub. But outside of that, their capabilities and user interfaces and so on may be substantially different. There are, for example, YouTube-like streaming video sites, GoodReads-like sites for tracking your reading, etc., and they can all communicate such that when you read and review a new book, your post about it might appear in someone else's Mastodon feed, etc.