Mastodon is its own thing. It’s a microblogging platform like Twitter. Lemmy is an aggregator and discussion platform like Reddit. Kbin is a mix of both.

But they all can talk with each other as they’re using a set of communication protocols that everyone understands, called ActivityPub. This protocol is the foundation that made the fediverse possible. It means content can be spread across many servers, and interactions can happen the same way.