All vehicles have an accessory belt that drives features such as the alternator and air-conditioning compressor, and perhaps the power steering and water pump. This belt is usually mounted externally so you can see it on the front end of the engine (the engine is mounted transversely on most front-wheel-drive vehicles), and it usually is a serpentine belt that slithers around various pulleys. It needs to be replaced when it shows wear, such as cracks, fraying or stretching.
Some engines also have a timing belt that is hidden from view but connects the crankshaft on the bottom of the engine to the cylinder head on the top; it controls when valves open and close. Many engines have a timing chain instead of a belt. A key difference is a timing chain is usually considered a lifetime part that doesn’t require periodic replacement. A timing belt does require replacement, though the manufacturer’s maintenance schedule may not call for that until as much as 150,000 miles.
Though accessory belts can last more than 100,000 miles, they should be inspected by a mechanic periodically (if you know what to look for you can do this yourself). Consult your owner’s manual or service schedule to find out how frequently it should be inspected; this varies by manufacturer.