The latissimus dorsi are the large triangular muscles that extend from under the shoulders down to the small of the back on both sides. These are the largest muscles of the upper body. Their function is to pull the shoulders back and down.
The spinal erectors are several muscles in the lower back that guard the nerve channels and help to keep the spine erect. They are also the slowest muscles in the body to recuperate from heavy exercises.
The trapezius are flat and triangular muscles that extends out and down from the neck and down between the shoulder blades. The traps work in opposition to the pulldown function of the lats, they raise the entire shoulder girdle.
The back muscles are essential to lift and carry heavy objects and to keep a good posture and a healthy mobility. Develop a strong back is absolutely necessary.
To plan an effective back training program is important to appreciate the complexity of the back muscles and how many movements it takes to get a full back development.
One of the reason that causes poor back development is poor back training technique. Back training is more subtle and more difficult than most people realize. For one thing the basic function of the lats and other back muscles is to pull the shoulder girdle down and back. If we don't pay attention, while performing back exercises, and we use the lower back or shoulders to pull, we'll never get to work through a full range of motion. The trick of back exercises is learn to isolate the back muscles, then make it more challenging instead of easy.
Source: "The New Encyclopedia of Modern Bodybuilding " (SCHWARZENEGGER, 1998)