Sound vibrations from the outside world are conveyed through this system until they reach the brain, and we hear the sound.
Here's How It Works [Diagram of The Ear]
Sound vibrations, or sound waves, are collected by the outer ear (those are the things hanging on the sides of your head!) and travel into the ear canal, where they bump up against the eardrum. The ear drum vibrates in sympathy with these sound waves. As it vibrates, it moves a series of tiny bones in the middle ear, which carry the vibrations to a fluid-filled tube called the cochlea in the inner ear. The fluid inside the cochlea vibrates a series of tiny hairs called cilia, which are attached to auditory nerves. The movement of these cilia stimulates the nerves, and they send signals to the brain, which, in turn, processes these signals into the sounds we hear.