Oh god, the gun debate rears its ugly head.

We used to have a more specific and equally flawed tier system, now we have a very general one, my advice for learning tier is simply to look at the other characters posted and scale your character appropriately. Ultimately it doesn't matter what tier your character is labelled as, so long as they are a fair match-up for at least a few characters so you have someone to fight.

Only tournament hosts have to worry about establishing a really specific tier categorization, and good luck to them as I think it's probably the hardest thing to do in Arena.