Federation is when instances exchange information. For example, you posted to lemmy.ml, while using lemmy.world, and I’m seeing this post and replying to you from lemmy.max-p.me. That’s federation in action, in this case with 3 instances involved. It runs on my server entirely in my control, I’m the admin of it. It’s completely independent from lemmy.ml and lemmy.world but they all talk to eachother.

An instance is one website running lemmy. Lemmy.ml and lemmy.world are two examples. There’s hundreds of them, all operating independently and sharing information.

Defederation is essentially when federation is cut off from a given server, think banning an entire instance essentially.

The fediverse is the name of this whole system: it’s federation and universe smushed together, fediverse.

Cross-platform instances would be instances that can talk to multiple platforms. For example, Kbin lets you browse Lemmy threads and Mastodon microblogs from the same place. Mastodon users can somewhat see Lemmy posts. I’ve never seen it referred to as cross-platform though.

But why are there so many instances and from what I have seen they are mostly similar to each other in terms of features and such? Like why would one use lemmy.ml over lemmy.world or vice versa when they are federated?

Because instances are hosted by individual hobbyist and not a company that makes profit, the amount of data that an individual instance can store and process is much more limited by their hardware. Spreading the load across numerous hosters allows for a higher capacity of data storage and therefore total lemmy users.

As for why one would choose a particular instance over another; you’ll find the each lemmy instance typically has its own social ecosystem and political consensus, because of this certain users will gravitate to the one that suits them best. For instance, lemmy.blahaj.zone is very LGBT heavy with an emphasis on the T, lemmy.world is quite liberal, lemmy.ml is typically socialist, hexbear.net is communist, and lemmy.dbzer0.com is pirate. While many of these are federated with eachother you will often notice an emphasis on certain subjects on an instance by instance basis