Dips thoroughly work your triceps and anterior deltoids, for a vigorous exercise. Dips also involve your chest muscles, the pectoralis major and pectoralis minor. If you don't have access to dip bars or aren't yet strong enough to do full dips on your own, bench dips also provide a vigorous workout for the triceps and chest.
Bench dips are usually done off the side of a weight bench. Any stable, firm horizontal surface will do, however, as long as it's tall enough that your hips don't hit the ground at the lowest point of the dip. Examples include a broad chair, a storage or seating bench at home, elevated flooring on a jungle gym and even the side of a firm bed.
Sit down on the edge of the bench or support you've chosen. Place your hands on either side of your hips, fingers overlapping the bench edge. Start by moving your buttocks off the edge of the bench, and walk your feet forward until you can lower your hips straight down in front of the bench. Lower your hips 2 to 3 inches and then press up with your arms to return to the starting position. If this is too easy, lower a little farther and press back up. Work your way up to where your shoulders are even with your elbows before pressing up to the starting position. Use your legs only as necessary for balance. Your torso should stay upright, hips below shoulders, and as close as possible to the bench throughout the range of motion.
For the easiest bench dip variation, keep both feet planted on the floor near your body. Squat down as you lower into the dip and then push with your legs to help yourself back into the starting position. To make the exercise slightly more difficult, extend both legs straight in front of you, resting on your heels. Your torso should still remain vertical throughout the movement but you won't be able to push as much with your legs, so your arms will take more of the weight. For a greater challenge, elevate your feet on a second bench, and cradle a weight plate or dumbbell in your lap to add additional resistance.
Bench dips work the same muscles as a regular dip, with the primary emphasis on your triceps and secondary involvement from your deltoids, pecs, rhomboids and lats. Doing bench dips regularly builds your strength to do regular dips using a dip bar. Once you can do a full set of bench dips with your feet up on a second bench, you're ready to try doing a few regular dips with your whole body suspended.