Dumbbells are inexpensive, versatile and easy-to-use weights that let you create a variety of workouts to improve your health. Depending on the amount of weight you use, number of repetitions you do and pace at which you use them, dumbbells can help you improve cardiovascular fitness, burn calories, build muscle or increase muscular endurance for sports. Dumbbells are small weights, shaped like a Roman letter I, resembling mini, single-piece barbells. You purchase dumbbells in different weights, starting with lightweight versions -- less than 2 KG -- to use during aerobic workouts or while using a treadmill or exercise bike. Heavier dumbbells let you bodybuild.