Everyone already mentioned that they’re federated and running on small servers. From a user perspective, Kbin is most similar to Lemmy as they are both similar to Reddit, but they are different in that they are completely different projects. Kbin seems to have fewer instances running right now, so everything on Kbin is a little more concentrated among the instances that do exist. But since you can access both Lemmy and Kbin from either, I’d stick with whichever one has the interface that you prefer.
Mastodon is similar to Twitter in how it looks and how people use it.