Lemmy is a software that people can host on their computer, and many people doing that form what is essentially a bunch of mini-reddits that can talk to each other to create one big platform.

Piefed is trying to fulfill the same goals as Lemmy, and is even fully compatible with Lemmy, so someone hosting a piefed server on their computer can join in with all the Lemmy servers, and to the Lemmy people, it appears to them like any other Lemmy server.

But underneath everything, the code base is entirely different. The commonality they share, along with mastodon, is they all use ActivityPub, which is the standard that allows them to all communicate and be compatible with each other, just like there’s an email standard.

Kbin (now Mbin) is yet another Lemmy compatible software that you can host on your computer, but it also tried to implement features that make it more like mastodon (twitter-like), so it can act both like reddit, with threads and comments and communities around single subjects, or be like mastodon and work with hashtags and following individuals instead of communities, like a microblogging website.

They also use different interfaces, but it’s only visible to people who directly use that server; to others who access it from their home server, it’ll adopt the look of the software their home server is using.

So as an example, you are using Lemmy since your home server is Lemmy.ml. if you visit a community hosted on a piefed server from within your Lemmy, like !fullmoviesonyoutube@piefed.social, it’ll look like any other Lemmy community.

But if you directly go to that piefed server by going to https://piefed.social/c/fullmoviesonyoutube you’ll see it from the piefed interface, since you’re accessing that piefed server directly.

All of three of the different federated Reddit-like softwares are intercompatible, so they all make up one big network.