The function of the alternator drive belt is to power the alternator, using the crankshaft pulley as the driving force.
There are two types of alternator drive belts. A standard, or V belt, is driven directly from the crankshaft pulley. The other, a serpentine belt, winds around various accessories like a snake, hence the name, powering the power-steering pump, as well as the alternator and water-pump.
An alternator drive belt should be tensioned properly for best performance and longest life. At the midway point of the belt, the deflection should be between a half inch and three-quarter inches when properly tensioned.
If the belt appears dry-rotted or displays cracking when flexed, it should be replaced. A glazed appearance on the inner surface is an indication that the belt is slipping and needs adjustment.
A vehicle's drive belts are the link between the crankshaft and engine accessories. Belts are often referred to as "drive belts," because the accessories that they power are "driven" by the crankshaft.