The way in which humans hear is rather complicated, and explaining it fully involves aspects of physiology, psychology and acoustics. A simpler description concentrates on the action of the ear as it converts sounds to a sequence of nerve impulses which are transmitted to the brain. The ear's ability to do this allows you to detect, all at once, a range of sounds, from loud to soft and from a high-pitched squeak to a low-pitched growl.
The ear consists of three parts: the outer ear, the middle ear and the inner ear. Each part serves a specific purpose in the task of detecting and interpreting sound.
The outer ear collects and channels sound to the middle ear. The middle ear transfers the to and fro motion of the air (which is the sound) into matching vibrations of the bone structure of the middle ear. These vibrations are transmitted into a to and fro vibration in the fluid of the inner ear. This vibration is changed into nerve impulses, which are then transmitted to the brain as electrical signals.
The board game
Mouse Trap provides a good basis for imagining how the ear works.