They are different platforms that can communicate with each other, and whose users can interact with, follow, and reply to each other. Mastodon is a microblogging service like Twitter, where everything is a short text post, and Lemmy is more of a link sharing/aggregation service where you can post links or text and others can comment on them. As far as I know, Mastodon users can comment on Lemmy posts, but their ability to post directly to Lemmy communities may be limited due to the platforms having a different feature set.

They both comply to the ActivityPub protocol, but they do very different things, as one is a microblogging platform while the other is basically an image board. You could say that what's different about them is simply their front-ends, but that wouldn't be correct, as each AP server allows their users to do different things. For example, you cannot publish standalone posts in "communities" (or "groups") via Mastodon, while Lemmy doesn't let you publish posts without specifying a community.

@nom_nom Different servers realize different services, or at the very least they realize them in different ways. What's new is that these services are at least somewhat interoperable with each other: I can comment on this Lemmy thread via Mastodon, altough I cannot start threads in a Lemmy community. Other AP servers work much better with Lemmy. For example, Friendica (a Facebook clone) allows for the publishing of posts in groups (which is what Lemmy communities basically are).

Very interesting, and thank you for the excellent explanation. I think I understand a little bit more now: so they are different services but because of the shared protocol it allows for some interoperability between services, and the unique part about the Fediverse is that it isn't simply an API, but all federated services (lemmy, mastodon, friendica, etc.,) are all literally interconnected, but the exact functionality depends on the service itself. Did I get that right?