Traps are one large about a diamond shaped muscle at upper back and neck. It starts at the top of your neck and spreads to shoulder attachment along the clavicle bone and goes down to about the middle of your back. It is one of the bigger muscles in our bodies. The trapezius or trap muscle is used to raise shoulders, rotate your shoulder blades and this makes it useful when lifting different things. If you feel that your neck is thin as a pencil and would like to get a little more thickness to it, exercising traps is one way to get more mass to it.
There aren't that many effective traps only exercises, but there are plenty of exercises that exercise traps along other muscles. The main point when exercising traps is to do controlled and relatively slow motions, this makes your traps really work out.