What constitutes a "Piercing" attack?

In game you can get an armor that is +15 to piercing. SAo what is a piercing attack? Arrows? Swords?Axes? or just melee attacks in general?

Just wondering if it would be better to go with that or the other type another armor gives you.

thanks

Normally, piercing = arrows (and spears, but there are no spears in this game)