First, welcome to the #Fediverse! I'm glad you found your way in the #ActivityPub-based “federation network”.

So, wait… what are these terminologies? And what exactly is the “Fediverse”?

Think of it like this:

Name: Web
Protocol: HTTP
Browsers: Mozilla Firefox, Naver Whale

Then you have the Fediverse:

Name: Fediverse / Fediverse Network
Protocol: ActivityPub (extra info: Before ActivityPub it was #OStatus; before OStatus it was #OpenMicroBlogging )
Browsers: #Hubzilla, #Friendica, #Calckey, #Rebased, #Akkoma, #Foundkey, #Meisskey, #Pixelfed, #BookWyrm, #PeerTube, #GlitchSoc #Pleroma, #Misskey, #Mastodon, #Kbin, #Lemmy, etc.

All these “fediverse browsers”, like Lemmy and Kbin, are commonly referred to as fediverse software. (extra info: Wikipedia calls Lemmy, Kbin, and similar answers-type or Threads-type software as “Link-aggregators”.)

There is no “Mastodon network”. There is no “mastoverse”. There is no “threadiverse”. There is no “Threadinet”, or whichever -verse / -network people are coming up with.

Any software that is using the ActivityPub protocol is part of the “Fediverse network”. Just like how any software that uses the HTTProtocol is part of the Web.

That clear so far?

Now, we go to your question.

Q: I moved to lemmy.world and have noticed that lemmy.ml has been popping up as another featured instance. What differs from lemmy.world vs lemmy.ml? Is .ml more active?

lemmy.world and lemmy.ml are called instances (the terminology used to refer to a fediverse server/service). These two are only two out of many fediverse instances available. In paritcular, these two instances are only two out of probably a hundred Lemmy-based instances available.

What differs between the two? As far as features and functionalities go, none. Since these two Lemmy-based instances are using the same software, they have the same set of features. The only difference would be if one instance fell behind in Lemmy version upgrades.

Since lemmy.ml is one of two flaship servers of the Lemmy software project, naturally it will get the bulk of registrations and content.

For example, before the Reddit Migration, there were already existing communities for many similar Subreddits, and many of those are in lemmy.ml and are very active. So, when Subreddits started to migrate, some chose lemmy.world and other lemmy instances. Which allowed lemmy.world to catch up with the number of users and number of active communities that lemmy.ml already have. AND, some of these are about the same topics/fandom.

So… yep, that's the detailed explanation. ^_^

I hope I wasn't confusing.