If the serpentine belt breaks on a car the alternator will quit charging and the water pump may not work, causing the car to overheat. Before the belt breaks it is good to inspect it for cracks on the inside where the ribs are located. Also if the belt is worn the topside of the accessory belt will look rough instead of smooth. In the picture you can see the arrow pointing to the belt tensioner. Depending on where this is located will judge how tough of a job it is to do. There is a breaker bar connected to the tensioner in this picture. The breaker bar is a 1/2 inch drive bar. Some tensioners will have a nut to place a wrench on instead of this square hole for a breaker bar.
Many cars today use timing belts instead of timing chains that were used in the past. Most import automobiles use them and some new domestic autos are switching to them. There are advantages and disadvantages to using timing belts. Timing belts are more quiet than a timing chain. This is good for high performance applications that use knock sensors to detect detonation. The lesser noise minimizes the background noise caused by a timing chain. The downside is that the timing belt should be replaced every 60,000 miles. A timing chain will generally never need to be replaced. Many times people will wait past 120,000 miles and their belt will break. If the timing belt breaks the car will stop wherever it happens to be. Depending on what type of car you own the timing belt breaking can be very costly.