Your latissimus dorsi, or lats, are powerful back muscles. Your lats adduct, extend, internally rotate and transversely extend your arm at the shoulder and also assist with scapular depression, downward rotation and adduction. To put it another way, if you're pulling something toward you, or pulling yourself toward something, your lats will be involved.
The pullover primarily works your latissimus dorsi, along with your teres major, rhomboids, levator scapulae, abs, pecs and triceps. If you only have time to do two or three upper-body exercises, this should be one of them, because it works the most muscles at once of all the bench-based exercises and requires very little equipment. All you need is a single heavy dumbbell or a barbell and a weight bench or, in a pinch, an exercise ball or even the bed to serve as a base for the exercise.
To do pullovers with a barbell, grasp the barbell overhand and lie down, face up, on a weight bench. Hold the barbell straight over your chest, arms slightly bent. Keep your elbows bent at the same angle as you lower the bar behind your head until your elbows are level with your head. Reverse the motion and repeat.
The bent-over row targets all your major back muscles: the latissimus dorsi, trapezius, teres major and rhomboids. While the bent-over row doesn't target your chest, triceps and abs the way a pullover does, it's a versatile back exercise because varying your grip and the weight position will shift the emphasis toward different back muscles. The more your elbows flare out, the more you'll emphasize trapezius involvement; the less they flare, the more you're putting emphasis on the latissimus dorsi.
Hold a barbell in an overhand grip and bend forward from the hips, back straight, knee slightly bent. Lift the bar straight up toward the middle of your torso, and then lower the bar until your arms are straight but not locked. Repeat.
Like the bent-over row, the dumbbell row works all your back muscles, but the characteristic elbow-close-to-body arm positioning of a dumbbell row emphasizes latissimus dorsi involvement more than the slightly-flared positioning of a bent-over row. You can also let your elbow flare out from your body as with the bent-over row, but you may find this more awkward, done one arm at a time with dumbbells, than a barbell bent-over row that works both arms at once.
Bend forward from the hips, supporting yourself on a weight bench with your opposite knee and arm. Extend your left arm, holding the dumbbell directly beneath your shoulder, palm facing in. Bring the dumbbell up and back, bending your arm at the elbow, then lower the weight and repeat.