Forum:Arena? What's that about?

sorry, this seems like a dumb questions since it seems like everyone knows what arena is.

Well, i don't.. I'm guessing you use it just to try your character against other players' characters?

I went into online and never saw a way to host a game in some kind of 'Arena mode' or anything like that nor have I seen an actual arena, is there something I need to do before I can use or enter or unlock something for an Arena??

I just want to know how to play in the arena and if there is something I should know about the arena. thx!!