There are a lot of Lemmy instances (or lemmies) out there. I don't even know how many my self 😅.

Each instances are characterized by their set of rules and moderation styles. You can see them through join-lemmy.org/instances as well as their description. To see a user's instance, just look at the domain after their username and the @.

Usually each instances can decide if they want to be able to interact with users and communities from a certain instance. That's called being federated.

Sometimes tho they don't think an instance's userbase or community follows their rules, so they can decide to cut all interactions with them. That's called being defederated.

The instance you are on, lemmy.world, is ironically, very US centered, and is the most popular instance. It's defederated from hexbear.net and lemmygrad.ml, so you will never see posts from them on lemmy.world unless it's a screenshot or something.

Since I'm on lemmy.ml tho, I can see posts from lemmy.world, hexbear and lemmygrad.

The benifit of this is that each instance can curate their experience how they see fit. If you think a moderator is being too trigger-happy on an instance, or you don't agree on how lenient they are (or whatever other reason), you can migrate to another instance. So let's say you want to go to lemmy.ml (which I highly doubt you would, but just an example), you go into your account settings, export your data, create your lemmy.ml account and import it. Currently it doesn't support importing comments, but the devs are working on it.

I'm not the best at explaining things, but I'm sure !newtolemmy@lemmy.ca could help.