This article is from the Piercing FAQ, by Anne Greenblatt with numerous contributions by others.
Nipple Retainer: Comprised of a straight bar worn through the
piercing which is held in place by a round or flat wire which
encircles the nipple. The bar is hinged to the wire circle.
Nipple Shield: Decorative jewelry which encircles the nipple and is
held in place by a barbell or ring.
Nipple Stretcher: Jewelry designed to stretch the nipple outwards,
usually held in place by a barbell worn through the piercing. One
design is comprised of a metal band or ring at the base of the
nipple attached by two or more legs to a smaller diameter ring over
which the barbell is worn. Another design is essentially a metal
corkscrew worn through the piercing which slowly stretches the
nipple outwards as it is rotated. Some nipple shield designs can
function as stretchers. Stretchers should only be worn on healed
piercings and the wearer should watch for signs of migration or
trauma to the piercing.