What is Elite? And can Elite players play with Regular playerS?

Elite players can only party with other elite players...

Elite mode is more difficult...More mobs, faster mobs, more champion mobs and less gold

Yeah, and when you sell items to vendors, they sell for 1/2 as much as in normal. And if i remember correctly, elite has better drops.

how does that work? is it a different server or something because all the mmos I've played, it's just one big server

You can see other normal and hc players, but you cant party with them, fight with them, talk with them, etc. But yeah, it's all one big server

You can talk and see players that do not play the same mode as you, you just can't party or trade with them