First, lets take a look at the main Arena page:
This should mostly be self-explanatory, but since the Universe keeps generating better idiots, it's becoming increasingly impossible to create anything that's considered "idiot-proof".
Your stats are on the left, and the system will randomly select an opponent for you and display their stats on the right. IT will also do a quick calculation to see what your chance of battle success is. You can still go for it if you want, but if you want to win, pay attention to the "Chance" area in the middle.
You can select a different player to attack if you like, and then the above calculations will update to the new player.
Note; You can't attack anyone who is already performing an activity (Academy, Pit, etc), nor can you attack anyone while you are performing an activity, either. This means you will have to attack before going off to the Pit, not the other way around.
If you win the fight, you will receive 250 of your opponent's Gold! If you loose the fight, you will give your opponent 250 of your own Gold instead.
Random System Attacks
A random attack will happen every hour, which is completely controlled by the system. You can only be exempt from RSA'a if your account currently holds VIP status.
New Cheevo's are introduced with this system, giving players who like to Cheevo hunt something extra to do to keep them out of trouble.. kinda. Being involved in an RSA is the only way to receive the achievement "With or Without Probable Cause".