A nipple shield is not usually nipple piercing jewelry in and of itself; it is an accessory for the piercing jewelry. It is usually a decorative plate with a hole in the centre. The nipple is placed through this hole, and then once the piercing jewelry is put through the nipple, the shield is held in place. Nipple shields are available in a wide range of styles. The nipple shield was a Gauntlet innovation.
Note that in general nipple shields serve an aesthetic rather than practical purpose, but nipple shields are available which push out the nipple and stretch it (see the Nipple Stretcher and Nipple Trainer entries). Nipple piercings saw a massive upsurge in popularity when Janet Jackson revealed hers on live television, prompting a humorous national furor.
More pictures of nipple shields can be found on BME in the nipple shield gallery.