Source: Player's Handbook
Choose a type of martial weapon, such as longbow (see Weapons for a list of martial weapons). You understand how to use that type of martial weapon in combat. Use this feat to expand the list of weapons you are proficient with beyond the basic list in the class description.
Benefit: You make attack rolls with the weapon normally
Special: Barbarians, fighters, paladins, and rangers are proficient with all martial weapons.
You can gain this feat multiple times. Each time you take the feat, it applies to a new weapon.
A cleric whose deity's favored weapon is a martial weapon and who chooses War as one of his domains receives the Martial Weapon Proficiency feat related to that weapon for free, as well as the Weapon Focus feat related to that weapon.
A sorcerer or wizard who casts the spell Tenser's transformation on herself gains proficiency with all martial weapons for the duration of the spell.
Normal: When using a weapon with which you are not proficient, you take a -4 penalty on attack rolls.