In ancient cultures, the ring was placed on the third finger of the left hand because it was believed that the vein in that finger led directly to the heart.
Because most people are right-handed, the custom is to wear wedding rings on the left hand. (This is mainly used to reduce the risk of damage or accidentally catching it on things.)
Catholics and many Christians (which include Protestants) wear it on the left hand, Jews (if I'm not mistaken) wear theirs on the right hand.It is said that you wear a wedding ring on your left hand...