The ear is the only organ in the body to detect sound.  It is made up of three parts; the outer, the inner and the middle ear, as shown on the picture below:

Sound enters the auditory canal as longitudinal pressure vibrations and causes the tympanic membrane (or eardrum) to vibrate in a transverse manner.  This moves the ossicles - a series of small bones called the anvil, hammer and stirrup.  The bones form a mechanical system, which gives an amplitude increase of 26 times.

The inner ear is made up of the cochlea, a fluid filled curling tube.  The stirrup pushes on the oval window at one end of the cochlea creating longitudinal pressure waves in the fluid.  These cause very small hairs to move, which send an electrical signal to the brain.