Dumbbells are essentially used in personal weight training, being the lightweight counterpart of Barbells (weights hanging from a bar). They are generally used in pairs, with one dumbell for each hand.
There is a real reason why they are called dumb. They were considered an equipment which simulated the action of bell ringing by pulling a rope, especially the strength needed, but without making such a noise. Believed to have originated in Greece and India, the versions used there were of a different shape than what is currently used.
There are many types of dumbbells, essentially three. Adjustable dumbbells consist of a metal bar in which plates are slid into from the sides and secured by clips or collars. Selectorized dumbbells are essentially similar, but the changing of weights is done by changing a dial position, on a dumbbell stand when the bar is kept on a stand. A third type, and an increasingly popular one in gyms is the fixed weight dumbbells. Their weights cannot be changed and come in denomination of one to five kilograms.
Fixed weight dumbbells are mostly made of iron, coated with neoprene or rubber to give them a smooth finish to hold. These are hence more popular among gym beginners. Experienced ones, however tend to prefer adjustable varieties.