Gonna give it a shot, so you know how there is reddit and it has communities and those communities have posts which have comments etc. Now imagine there are 2 reddits, Addit and Bddit both are exactly same in infrastructure/software stuff but obviously different users, communities and content but since the infrastructure is the same, both Addit and Bddit decided that it would be cool if user@addit could interact and do reddit stuff on Bddit and vice versa. The software that does this is lemmy, anyone can make a reddit like website with this and all users of that website will be able to interact with every other lemmy website. So lemmy is a group pf reddit. Now as you can imagine, website owner can decide which other website can interact with their website, this interaction is called “federating” so defederation means that blocking a certain website (also reffered as lemmy instance), as an example, addit could block bddit or vice versa so users of addit can not interact with bddit.

Now the concept of fediverse is also similar, in that case fediverse is a goup of fedi services, think a group of lemmy like services where each service has a group of websites/instances. This is possible because most of these services have similar things inside them, like each will have users, posts, comments etc so user@lemmy_instance@lemmy can interact with user@mastodon_instance@mastodon. All this is govern by ActivityProtocol. Honestly i don’t know to what extent these interactions are possible.

Sooooo in short lemmy= network/group of reddit like websites Fediverse= network/group of lemmy like services

All the Lemmy (and Kbin) servers (instances) can theoretically talk to each other. You can follow communities on other instances and comment and post like you are used to.

You could think of each server as an independent city-state, each with it’s own leaders, customs and citizens.

If two instances have a feud that can’t be resolved, they will “defederate” and then their users will no longer be able to contact each other.

There are Nazi instances here in the Fediverse (for example), but they get blocked on sight so you almost never see their garbage in the feeds.

Does that help?

When you think about it with 3 instances, it’s fun and strange.

Imagine 3 instances. lemmy.world / beehaw.org / lemmy.dbzer0.com.
Now call them A, B and C.

Now A and B defederates.
Accounts on A can’t see content published on B anymore. And can’t publish on these “forums” (communities/magazines…)
And accounts on B can’t see content published on A anymore. And can’t publish on these forums.

However, an account on C can see everything and publish anywhere !

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You should consider your account as a passport. The instance/country delivering you the passport is important, since it gives you permissions/visas. If everyone in your instance is behaving like shit, your instance will be defederated and you’ll loose permissions/visas to be able to browse other instances.

The other comments explain it well, but if you’re still confused, the easiest way to understand it imo is to just think about it like emails. Gmail, Yahoo, Outlook, Proton, these are all completely different websites, but they serve the same purpose and can interact with each other regardless. You don’t need a Gmail account in order to send your friend on Gmail an email. On the other hand, you can’t sign into Gmail with your Yahoo account.

Which one you sign up for is usually down to personal preference regarding their rules and website design, though this isn’t always the only reason to pick. If Google one day comes out as being run by Nazis, some email providers might choose to “defederate” Gmail, essentially blocking the ability to receive emails from them. If you don’t wish to receive emails from Nazis, it’s probably a good idea to pick one of them over one that doesn’t block Gmail.

Just imagine that but for a website like Reddit and you’ve got Lemmy.