In order for the engine to transfer rational energy to various components, like the alternator, there needs to be some way to connect the crankshaft pulley to the pullies of the other components. The serpentine belt is what makes this possible.
Serpentine belts have a variety of patterns which help keep it attached to the pulley system, but in almost every example, one side of the belt is completely flat, and this side usually faces towards the outside most of the time.
Some vehicles have multiple belts, so the crankshaft pulley can have two different sizes, which keeps the component pullies from having to be quite large. Sometimes custom sizes can be made so someone modifying their vehicle can bypass non-crucial components like the air conditioning pump or power steering.