Why YSK: Because intuitive explanations are few and far between, and the technical explanations often present too many "trees" and not enough "forest", which is just how technically-minded people are trained to approach things. Forests are, after all, made of trees, and it's not their fault we don't care about individual "trees". This then, is my unprofessional attempt to consolidate everything I've read over the past three days into one, easy-to-understand explanation of how all this shit works in lay-person's terms. Due to my amateur background, I may have details incorrect, and I would request that anyone who catches anywhere where I have made a mistake, even a small detail, to please correct me. I will also include a few links to my best sources at the bottom. tl;dr style explanations will be included after every paragraph in parenthesis. So, let's begin:
Imagine you have reddit. Fantastic, it's a giant forum composed of a whole bunch of smaller, sub-forums. But let's take this one step further. Why have just one reddit? Why can't we have lots of reddits, each capable of having its own complete set of subs, where each reddit is independent of every other one and has its own web address? Okay, let's do this, and push it to the extreme. Let's make it so everyone can make their own reddit, even individuals. So you, if you wanted, could set up your own complete reddit, with just you in it. You could have all the subs, r/TIL, r/TIHI, r/pics, etc etc, all with just you in them. You have total control! But you have no content and are probably pretty lonely, right? We'll get to that. Let's call this Self-Hosting though.
(So, we now have a situation where many whole reddits can individually exist, each in the vacuum of space.)
Now let's fix that content and loneliness problem. What if we allowed each reddit to communicate and share content with every other reddit, similar to how subs can communicate with each other? Boom. We just created a spider-webbed network, of countless individual reddits, each composed of subs, that can now all share content back and forth. Let's call this big spiderweb an over-reddit, to contrast it with subreddits.
(Now instead of a two-tier system of isolated reddits and their subs, we have a three-tier system, of over-reddit [the "Lemmy-verse"], reddits [Lemmys or Instances or Servers], and subs [communities or sub-lemmies].)
But, we actually have a technical problem. How do these individual reddits find each other? How do they know the other ones even exist? They could be on servers on opposite sides of the planet, with random web addresses. Obviously we can't just guess. So, okay, let's let users solve this for us via crowd-sourced labor. We don't have to find all the reddits for them. Let's just design the system so that the reddits only find out about each other after any random-ass user introduces them to each other. We'll call this batching, they can do it with the reddit search bar. Then, we'll wait for that random-ass user to actually subscribe to any new sub/community over there, which they'll only do if it's any good. Once this is done, now the two reddits and that one sub become connected, not just for that user, but their whole reddit userbase too. The rando doing the search and subscribing simply introduced two good reddits to each other. Now that they know about each other though, they'll share content back and forth freely, with comments, votes and posts all being visible to both reddits. Let's call this "federating with each other". It's not too different from neurons in the brain reaching out to each other, really.
(To find and connect the disparate, scattered reddits into our over-reddit, we use crowd-sourced labor.)
Well, that's it. That's the Lemmy-verse. But what about this Fediverse? Well, okay, remember what we did with reddit, and giving it a third tier of over-reddit? Let's do the same exact thing with twitter, facebook, youtube and every other thing we can pull out of our asses. Let's let all of them share and access each other's content with the exact same structure and system, so now you, hanging out in your reddit, can get all the tweets too. We've made a fourth tier now. The Fediverse, which is most comparable to the internet itself, and includes the Mastadon-verse, the PeerTube-verse, etc etc.
(Why stop there...? reddit is chump change, let's just do this to everything.)
So, that's it in a nutshell. That's how this shit works. And the next time someone says it's like email, I'm going to climb through their computer screen and smack them. It's only like email at that technical, "trees" level, and when you go up to the more intuitive "forest" level, this just serves to confuse the ever-living hell out of everyone.
(I'm a bit of a dick.)
One last detail: Admins can whitelist (allow-list) or blacklist (shadowban) other Instances/servers. As an example, one of the other largest Instances has blacklisted (shadowbanned) us here at lemmy.world, because we were producing too much spam. As a result, until they undo this, all of us here are shadowbanned from their Instance/server. We can see their content, they can't see ours. This enables them to control how much connection they have to the rest of the Fediverse.
(Let's not forget to give admins the power to stop people from other places bothering them, if they do not approve of the content. Very important feature.)
Sources: https://join-lemmy.org/docs/users/01-getting-started.html https://en.wikipedia.org/wiki/Lemmy_(software) https://github.com/amirzaidi/lemmy https://news.ycombinator.com/item?id=36387939
Again, if I've made any errors, regardless of how small, please let me know below. This is intended to be another reference material for lay-people, so accuracy is important. However, outside of major errors, I will not be editing this post to correct it, as I would prefer any corrections to be delivered from the full perspective of someone's individual expertise, instead of being translated into my own words.
(I don't actually know what I'm talking about. Scroll down for people who do.)
Hope this helps.
edit for grammar/cleanup