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

Just joined and I'm looking around for communities. I went to search for an Android one and I saw an Android one but also [email protected]. Is that a specific instance's Android?

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

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