Kbin is another federated instance on the verse just like Lemmy, it just has a different UI. I'm browsing this thread and commenting through kbin right now, we can all interact with the same posts :)

kbin.social is like lemmy.world, etc.... It's another instance (though maybe/probably there are other technical differences I don't understand). I am speaking to you from kbin right now. It's magic, you see.

Kbin is a separate application that is like lemmy and they can communicate with each other, as in view each other's content.

You are participating on a kbin thread right now, kbin is basically another lemmy but because they speak the same language we can communicate.

Think about email in comparison to for example Facebook. On Facebook, the content and the platform are to sides of the same coin: the two cannot be separated. In email, on the other hand, the two are separate: the content (emails) is separate from the provider (Google, Protonmail, or some old laptop with an internet connection functioning as a server). Emails can be sent between the different platforms, as it is based on a protocol that exists independently of the providers.

Lemmy is the same way. In the example above, Lemmy is a piece of software you could install on your old laptop (or other people can install on hopefully more sophisticated servers) in order to communicate with other services. That's why Lemmy can be found on multiple websites (lemmy.world or beehaw.org, among others). Just like in email however, the protocol is separate: the protocol Lemmy runs on is called ActivityPub, and works with not only Lemmy, but also other software. Mastodon is the most famous; kbin is another.

Like Lemmy, kbin is a client for ActivityPub that is made to function a lot like Reddit. It is developed completely separately, but according to similar logics: I saw your message in kbin and am responding to it from there, and when I upvoted your post you recieved an upvote in your Lemmy. It's similar to if you are using Outlook and I am using Gmail: I can still send you an email and communicate effortlessly across the two services.

The difference between kbin and Lemmy is mostly in user experience. Both projects are work in progress, development is happening fast, and they have both seen an explosion in the number of users the last few days. Content spreads between both freely, so the choice between them is really mostly about user interface preferences. It will be fun to see how both projects develop.

If you look at the links by each post, you'll notice that some will reference a URL that goes off of your local instance. In Lemmy these are icons, in kbin it appears from the "more" link. Sometimes it's unclear who/where I'm interacting with and examining the URL helps me get some idea of it. In federated social media different instances often develop a different subculture, but since they can access each other you have more dimensions of interaction and how to behave.