In Western cultures a wedding ring is traditionally worn on the
ring finger. According to tradition, the wedding ring is worn on
the left ring finger because the vein in the left ring finger was
believed to be directly connected to the heart, a symbol of love.
This belief is thought to have originated in ancient Egypt.
In medieval Europe, the Christian wedding ceremony placed the ring
in sequence on the index, middle, and ring fingers of the left
hand, representing the trinity God the Father, God the Son, and
God the Holy Spirit. The ring was then left on the ring finger. In
a few European countries, the ring is worn on the left hand prior
to marriage, then transferred to the right during the ceremony. A
Greek Orthodox bride wears the ring on the left hand prior to the
ceremony, then moves it to the right hand after the wedding.
In Norway the wedding ring is worn on the ring finger on the right
hand.
In the Jewish wedding ceremony, the groom places the ring on the
bride's index finger, and not ring finger; the ring is usually
moved to the ring finger after the ceremony.
http://en.wikipedia.org/wiki/Ring_finger#The_wedding_ring
In the British-American tradition, an engagement ring is a ring
worn by a woman on her left-hand ring finger indicating her
engagement to be married. By modern convention, the ring is usually
presented as a betrothal gift by a man to his prospective bride
while or directly after she accepts his marriage proposal. It
represents a formal agreement to future marriage.
http://en.wikipedia.org/wiki/Engagement_ring
the stuff that wiki has.. 
