Sound is created when an object vibrates. These vibrations are perceived by our ears and then sent to our brain. Our brain in turn analyzes these signals and let's us know what type of sound we are hearing (i.e. an alarm clock ringing, a car horn blaring, etc.). Music is differentiated from other sounds because it has certain qualities.
When you listen to a piece of music, you'll notice that it has several different characteristics; it may be soft or loud, slow or fast, combine different instruments and have a regular rhythmic pattern.