Dumbbells is a short bar with disk-shaped weight at each end, 10 to 15 inches long. Dumbbells are designed to be help in one hand. 

Benefits: Dumbbells offer an increased range of motion for some exercises. Dumbbells also allow you to work your body unilaterally, one side at a time. If your left bicep is weaker than your right, you can focus on just the left side with dumbbells.