What's the difference between two communities when one has an @ and the other doesn't?

The one without the @ part should be the community from your instance (e.g. if your account is in lemmy.ml then it is [email protected]). Since it is local, they just do not display the @ part

Communities without @ are those created on your local server, those with @ are created on other servers.

Yes, subbreddits on lemmy are called Communities, that's why /c/ instead of /r/.

Edit: I may have figured it out. The instance I’m on doesn’t show the @ but other instances do. Is that correct?

Exactly

Also minor question, are they called communities here? And it’s /c/ instead of /r/ right?

Yes.

I recommend subscribing to the communities you're interested in so you can see the posts only from those. It also keeps you from confusing communities with the same names from different instances. There's no curated front page here, unlike reddit, so it's good to curate your own experience.

please use [email protected] for support questions.

The @ introduces the instance address. Just like in email addresses. inbox@domain. On lemmy it's community@domain.

A local instance may omit the @self, but works with it too.

RE: Your edit, you're correct. Domains aren't included for local communities.

One is local to your instance, the other is hosted on another federated instance.

Yes on all accounts.

[email protected] is the Android community on the whatever.com server.

They are generally called 'communities' on Lemmy, I believe they are called 'magazines' on Kbin.

The instance you are logged in to won't show the @whatever.com if it has an Android community.

You're right, things that are local to your instance don't have any qualifiers on them. As another example of that my username should show up with the instance I'm on since it's different than the one you're on