Engagement and wedding rings are traditionally worn on the fourth finger of the left hand in many Western cultures, including the United States and much of Europe. In India, Spain, Peru, Norway and Denmark, couples wear rings on the fourth finger of the right hand.
The tradition of wedding and engagement rings on the fourth finger goes back to ancient Egyptians who believed a nerve ran from that finger straight to the heart. Greeks and Romans also believed this, and they called it the "vena amoris." While the tradition remains, it is not anatomically correct.
More practically, the ring finger is one of the least-used fingers on the hand, and most people are right-handed. Therefore, an engagement ring generally receives less damage on the fourth finger of the left hand.