Increasing muscle mass and functional strength constitute the two main motivations for adopting a bodybuilding program. Building abdominal muscles, specifically the muscles of the upper, middle and lower back is the key to both of these.
The primary stabilizing muscles of your body are in your back, these are the easiest muscles to injure while lifting weights. Your back consists of the spine, the ribs, the scapula and the ligaments that string them all together, like parts of a mobile or kinetic motion structure.
Injuries to the back are almost always compression or rotation injuries, they're damage to the soft tissue that lets your back move. Good weight lifting practices and muscle group isolation are the key to avoid back injuries, many of which won't show up for years after you've done them to yourself.
Traps, Lats, and Deltoids are some back muscle groups to work on, but take your time. Doing it slowly will prevent injuries. Sure, you are going to get a ripped back by overworking, but you should concentrate more on everyday strength over getting to big when working your back.
The traps muscles are the ones that form the bulk of your back. If you are looking to do Traps, then use a shrug motion, which has you taking dumbbells in each hand, and shrugging your shoulders up and down while you take a breath and then let them down while exhaling.
A related motion, which helps the traps a bit, but mostly works on your deltoids, is a straight-arm lift. Hold a dumbbell in your hand with your elbow at your side, and your forearm out level with the map. Extend your arm until it's at shoulder height, in one fluid motion, and look down your arm at the dumbbell, like you're sitting on a pistol range. Then bring it down slowly.
When you do any kind of exercise to build up a muscle you should always start out slow and build your way up to more and more resistance. If you start to fast and do to much at once, you can end up hurting yourself before you even get started. This is better than a straight pull up because it isolates the muscle thoroughly.
Building definition, and most importantly, strength, in the lower back requires shoulder arches. Hold a light dumb-bell over the back of your neck and lay down on your front. With your hands clasped, arch your back, lift, and hold for a count of three, then lower slowly. Do these sparingly; it's very easy to overdo them.
It is very important to remember to stretch before exercising. By stretching you will help your body and your muscles maintain flexibility, along with stretching it is very beneficial to also do some cardio exercises as well.