The auxiliary belt is also known as the serpentine belt, and shouldn't be confused with the timing belt which serves a completely different purpose.
The auxiliary belt's role is to 'power' a variety of important peripheral components via a system of pulleys driven by the main crankshaft pulley.
On old cars the belt simply connected the crank with the fan, so as the engine rotated so did the fan. But as engines became more complex, and fans became electrical rather than mechanical the fan belt was superseded by the V-belt, which as the name suggests is a V-shaped belt that powers one or more additional components.
It's not uncommon to find more than one V-belt depending on the application.
In more recent years the aux belt has become the preferred choice. Its wide, flat, often toothed design means that it can power several different components at once and has become the norm in engine bays where space is restricted.
On a regular front-wheel drive car you'll see the aux belt down the side of the engine (access may be very tight). The aux belt is virtually always visible, unlike the timing belt which is much more likely to be 'hidden' behind a cover.
It's not uncommon for the belt to power the alternator, power steering pump, air-con pump, and water pump. As the belt needs to be really quite long you're also likely to find an idler pulley, and/or a tensioner pulley in the mix.
These keep the belt correctly tensioned so it doesn't deviate from its course, and so that it applies sufficient friction to the pulleys.
The belts are made of reinforced rubber and are usually 'toothed' on one side. It's interesting to note that as the belt snakes its way around the various pulleys both the toothed side of the belt and the smooth side apply friction to the different pulleys depending on the level of 'load' they place on the belt.