Power Attack

Prerequisite(s): Str 13+
Benefit: Before you make a melee attack, you may invoke a penalty to your to hit roll, up to a maximum of your level. If you hit with this attack, you do that many points of extra damage. If you are wielding a two handed weapon, or a single handed weapon with two hands, the bonus damage is doubled. You canít add the bonus from Power Attack to the damage dealt with a light weapon (except with unarmed strikes or natural weapon attacks), even though the penalty on attack rolls still applies. (Normally, you treat a double weapon as a one-handed weapon and a light weapon. If you choose to use a double weapon like a two-handed weapon, attacking with only one end of it in a round, you treat it as a two-handed weapon.) A fighter may select Power Attack as one of his fighter bonus feats.
Source: Players Handbook