Pullovers work your chest, back, shoulders, triceps, and abs, as well as other stabilizer muscles.
To do a pullover, start by lying down on a bench, with a dumbbell over your chest.
Lower the weight back behind your head in a slow, controlled fashion while taking a deep breath in. Exhale through your mouth while pulling the weight back up and over.
If you want to emphasize more of your chest and triceps, keep your elbows close together; to put more emphasis on your back muscles, keep your elbows out wider.
I usually like to do pullovers with a dumbbell, using an open grip, which involves putting both hands flat against the bottom of the dumbbell and curling your fingers around the edge. Pullovers can be done with a barbell as well, which allows you to experiment with a wider grip.
Pullovers are typically performed on a bench, although you can do them on a stability ball to add more of a core challenge.
If you are more interested in putting mass on your upper body, then the bench is the way to go. When you do a pullover on a ball, you will have to use a lighter weight to account for the added stability factor, which makes the focus of the exercise less on building raw strength.
Pullovers might not be appropriate for people with certain types of shoulder injuries. Check with your trainer or doctor if you have a history of shoulder problems before doing this exercise.