The upper back consists of a lot of different muscles. Weight training exercises for the upper back will greatly improve the width and thickness of your body. For this reason it's important to train your upper back hard while following a weight training routine.
The most well known muscles of the upper back are the latissimus dorsi, or lats. These are the muscles that extend from the middle of the back to the sides of the upper back next to your arms that will make your upper body look wider when weight trained. The rhomboids, teres major, and infraspinatus are the muscles that make up the middle of the upper back. These muscles, along with the lats, will give your back more thickness when weight trained.
To train your lats for more body width you need to do pull-ups or cable pulldowns during your back weight training routine. Pull-ups are a superior back exercise to pulldowns because you are move your body through space as opposed to pulling a weight towards you. If you are too heavy and not strong enough to do many pull-ups, then pulldowns are a good alternative, and you can start doing more pull-ups as your upper back strength increases. Make sure to do close underhand grip pull-ups or pulldowns as well as medium and wide overhand grip pull-ups and pulldowns for optimal results whie weight training your upper back. Try different grip ranges to keep your body guessing.
To train your upper back for more thickness you must do rows. Rows can be done with barbells, dumbbells, cables, and machines in your upper back weight training routine. For barbell rows, bend forward at an angle (around 45 degrees to avoid putting too much pressure on the lower back) and pull the weight to your midsection, making sure to contract your back muscles hard. You can do these with an underhand or overhand grip. One-armed dumbell rows are done by putting the knee opposite to the side of the upper back your rowing with on a bench for support and pulling the dumbell from the floor to your side. Cable rows are a great weight training exercise for back thickness because you can use a lot of different grips (close, medium, and wide) to hit your upper back muscles from a different angle.
Don't rely too much on pulldown and row machines in your upper back weight training routine because they force you to pull the weight in a fixed plane of motion. If you're going to use machines, use them at the end of your uper back workout.
For many weight trainers, especially beginners, upper back weight training is easy to neglect, at least in comparison to the amount of chest training they do. One reason for this is that you can't actually see your back muscles working while you're training them. You must train your upper back muscles at least as much as your chest muscles, if not more, to keep a balance in your upper body. If you train your chest too much and your upper back not enough, you may end up front heavy which is bad for your posture and lower back. Don't neglect upper back weight training.