What exactly is the fediverse?

Servers that can federate over ActivityPub protocol. Any server that uses ActivityPub can be considered part of the fediverse.

What’s included in it?

Clones of corporate owned sites: twitter (mastodon, misskey), tumblr (wafrn), reddit (lemmy, piefed), facebook (friendica), plus others

I’ve hear much about fediverse and Lemmy, but is Lemmy part of it or not?

Yes, it is

Are other systems like Blue sky a part of it or not?

Bluesky isn't, since they use a different protocol, but it's possible to bridge and interact with it. Wafrn does it.

Do I transparently see posts from all those different systems?

This is the biggest "it depends" situation. For instance, by default, lemmy ignores most posts from mastodon and similars. However, mastodon users can post to lemmy communities if they use the proper @, but they cannot specify a title - their post body will also double as the title. They can also reply to comments, they're easy to spot because they always have an @user when replying.

Some server types integrate the different things better than others. Friendica and Wafrn seem to be the best for "variety integration".

You asked five questions, none of them are stupid.

What exactly is the fediverse? What's included in it.

I have heard two definitions in use. The first is narrower, it refers to the collection of servers running compatible Reddit-alike software including Lemmy, Mbin and Piefed which are pretty much 1 to 1 compatible and communicating with users on one from another is more or less seamless. The big, distributed Reddit alternative that allows you to post from lemmy.ca onto lemmy.world and me to read it from sh.itjust.works.

The second is the broader, simpler definition of "anything that runs on the ActivityPub protocol and is federated with something else." Which includes all of the above plus the likes of Peertube, Mastodon, Pixelfed, Loops etc. They are technically cross-compatible, I'll get to that later.

Is Lemmy part of it or not?

Yes it is, Lemmy runs on ActivityPub.

Are other systems like Bluesky part of it or not?

Some are, some aren't. A few examples:

• BlueSky. Not part of the Fediverse, it uses a different protocol, their own thing. It is sort of designed to federate but not really in practice.
• Diaspora. Similar concept of federated social media, but not compatible with ActivityPub. The Coke to our Pepsi.
• Truth Social. It is my understanding that The Church Of Trump is basically a fork of Mastodon. They don't federate though, they turn that feature off thank a long list of random deities and WWE wrestlers.
• Threads. Meta/Facebook's Twitter clone. IS part of the Fediverse, it uses ActivityPub and has federation turned on, though a lot of instances defederate with them on principle. You can interact with Threads from a Lemmy instance. ...If it still exists. Is Threads still a thing?

Do I transparently see posts from all those different systems?

Yes and no. You can kind of think of the Fediverse like the Universe itself in that there's nowhere you can stand and see the entire thing. You and I are from neighboring star systems in the same galaxy, we're both on servers running Lemmy, so we can communicate completely seamlessly. I see a comment immediately above you from someone on piefed.social, they're on a server running Piefed, not Lemmy. That's another Reddit-alike, they can communicate with us pretty easily. You might occasionally see someone on Mastodon chime in. You can usually spot this because they @ the users they're replying to. It would be really cool if a Mastodon user could reply to this message to demonstrate. As you get farther afield, it kinda stops working. It's difficult to interact with Peertube from Lemmy, for example. I have commented on a Peertube video from a Pixelfed account though.

Not a stupid question at all. Loosely, I think, it's any site that trades information using the ActivityPub protocol. Because they use the same underlying protocol, they can easily trade content/posts with each other and yes, Lemmy is part of the Fediverse for that reason.

This is also why you can see posts from lemmy.ca or piefed.social or whatever.domain users while browsing lemmy.world - anyone who sets up a site with the Lemmy software can participate in the network and trade posts with all the others - these are individually called instances. These sites can decide that they don't want to trade posts with certain other sites (ie: trolls set up a farm on their own instance) and exclude them from their users being able to see them, this is called defederation.

In theory, a Mastodon instance could see content from a Lemmy instance (and Pixelfed and Loops and so on) as they all use the same underlying protocol to trade information, but in practice, it seems that sites basically stick to trading with other sites in their wheelhouse.

BlueSky also started with ActivityPub but I believe they did something to their software to make it proprietary.

The usefulness of all this is: no member site can get a monopoly on content. The largest Lemmy site is lemmy.world and I have an account on there. I switched to dbzer0 because I disagreed with some of the actions taken by lemmy.world (they defederated from some content that I wanted to see) so I came over here and now I can see that content.

Anyway, that's my understanding of it.

The fediverse is the overarching architecture. So lemmy is part of the fediverse. It is a federated collection of servers that are somewhat independent, but still part of the system. I'm going to make a very poor analogy, but think of it like your spice cabinet. The individual spices are instances while the cabinet itself is the fediverse. Or at least that's my understanding of it. If I'm mistaken, please someone enlighten me.