How Hearing Works
When sound waves enter the ear, they travel up the ear canal, and hit the ear drum. The ear drum vibrates, and allows the three smallest bones in the human body to start working. These bones (ossicles), cause vibrations in the fluid in the inner ears sensory organ, the cochlea. The vibrating fluid causes sensory hair cells to vibrate, which in turn send nerve impulses to the brain. The brain translates these impulses and we perceive sound.
Helpful Links
For more information visit:
Sounds Amazing - A good reference guide to how we hear. Has lessons on sound and hearing.
Sound (Wikipedia) - The Wikipedia article on sound.