You are especially skilled at using weapons that can benefit as much from Dexterity as from Strength.
Prerequisites: Base attack bonus +1.
Benefit: With a light weapon, rapier, whip or spiked chain made for a creature of your size category, you may use your Dexterity modifier instead of your Strength modifier on attack rolls. If you carry a shield, its armour check penalty applies to your attack rolls.
Special: A Fighter may select Weapon Finesse as one of his fighter bonus feats.
Natural weapons are always considered light weapons.
Source: Player’s Handbook (Page 101)