I'd explain it in full detail, but I've got a bit too much on my plate to write it up.

The rough summary is: 
The Gamemaster dictates how the roleplay is decided, whether character tiers are listed or not is up to them. Character tiers are a VERY ROUGH AND VAGUE measurement of the power level of the characters within the roleplay. 

There are two universally accepted rules: (Some others are unspoken, and some are contested. These are the two that are uncontested.)
No Character Control: The act of performing an action through someone else's character without their permission.
No God Modding: The act of making your character do something that would be impossible for them.

Arena Roleplay is a turn based thing, you take a turn and they take a turn. Your turn is in response to theirs. Your action can be interrupted if logically they can react first.

So on and so forth, it's real simple.