There are usually no real mechanics. The combat is done arbitrarily, with player 1 describing an attack and player 2 getting to decide whether it hits them. Some are more strict about the separation into turns and what you can and can't do in a turn than others, but it's pretty intuitive either way.

Tiers are a way to indicate your character's power level. The most recent tier system is the one from LeeRoy's [url=http://www.roleplayerguild.com/topics/63723-the-everlasting-arena-character-deposit/ooc]Everlasting Arena[/url] (bottom of the first post), but there are several older ones that people may refer to. Knowing your character's tier is important in competitive threads, but not so much for this.

Edit: Vordak, you ninja :P