All sound is just a series of vibrations in the air. That's why there's no such thing as sound in outer space (no air), and that's why sound travels faster through solids or liquids (there's more stuff there to vibrate).

Our outer ears, the part you can see, collect and focus those vibrations. In the middle ear, the eardrum is a tight membrane that moves with those vibrations. The eardrum is connected to three little bones called the malleus, the incus, and the stapes. They move as the eardrum moves,transmitting the vibrations to a fluid-filled chamber in the inner ear. As the inner ear fluid vibrates, little hairs called cilia move in the current. Each hair or cilium is connected to a nerve that goes to the part of the brain devoted to hearing. Our brains translate the nerve impulses into what we perceive as sound.

We can detect the direction of the sound source if the sound arrives at different ears at slightly different times. We perceive loudness and pitch in sounds based on characteristics of the vibrations. "Bigger" vibrations,or sound waves with greater amplitude, are perceived as louder sounds.

"Faster" vibrations, or sound waves that occur with greater frequency, are perceived as higher in pitch.