The serpentine belt in a car operates the water pump, which keeps engine temperature under control; the alternator, which charges the battery; power steering; and air conditioning. The serpentine belt gets its name from the way it winds around all the pulleys driven by the crankshaft, replacing the older V-belt system of using individual belts for individual accessories. This makes replacement easier, but when it breaks, everything it is wound around stops rotating. Many serpentine belt systems include a spring loaded pulley known as an automatic tensioner, which ensures that the belt is not too tight or too loose. It is recommended that these belts are replaced every 45,000-60,000 miles or when it becomes cracked, frayed, or glazed.
info from http://www.lubefast.com/serv05.htm and http://www.oilchanger.com/automotive/serpentine.htm and http://autorepair.about.com/library/glossary/bldef-728a.htm