Taken directly from the SRD (references to spellcasting and casters removed)
NONLETHAL DAMAGE Dealing Nonlethal Damage: Certain attacks deal nonlethal damage. Other effects, such as heat or being exhausted, also deal nonlethal damage. When you take nonlethal damage, keep a running total of how much you’ve accumulated. Do not deduct the nonlethal damage number from your current hit points. It is not “real” damage. Instead, when your nonlethal damage equals your current hit points, you’re staggered, and when it exceeds your current hit points, you fall unconscious. It doesn’t matter whether the nonlethal damage equals or exceeds your current hit points because the nonlethal damage has gone up or because your current hit points have gone down. Nonlethal Damage with a Weapon that Deals Lethal Damage: You can use a melee weapon that deals lethal damage to deal nonlethal damage instead, but you take a –4 penalty on your attack roll.
Lethal Damage with a Weapon that Deals Nonlethal Damage: You can use a weapon that deals nonlethal damage, including an unarmed strike, to deal lethal damage instead, but you take a –4 penalty on your attack roll. Staggered and Unconscious: When your nonlethal damage equals your current hit points, you’re staggered. You can only take a standard action or a move action in each round. You cease being staggered when your current hit points once again exceed your nonlethal damage.
When your nonlethal damage exceeds your current hit points, you fall unconscious. While unconscious, you are helpless. Healing Nonlethal Damage: You heal nonlethal damage at the rate of 1 hit point per hour per character level.