In standard OU battles, the rules are as following:
Sleep Clause - This rule ensures that you are only able to send one of your opponent's pokemon asleep at any given time. You may send a new pokemon to sleep once the first pokemon wakes up or is fainted. The opponent is under the same restriction.
Freeze Clause - This rule works the same way as Sleep Clause
OHKO Clause - Any one-hit ko move used will fail in battle, and will count as a waste of a turn.
Evasion Clause - Double Team and Minimized can be used, but will fail in battle, and will count as a waste of a turn.
Species Clause - Neither battler is allowed more than one of the same pokemon.
Soul Dew Clause - No pokemon is allowed to hold the item Soul Dew.
Timed Battle and Strict Damage Clause are pretty much givens, though.
Anything deemed as uber is banned.