Here's a simple overview of how the ear works.
Your ear is made up of three parts – these different parts are often referred to as the outer ear, the middle ear and the inner ear.
The Outer Ear
The outer ear starts with the bit that sticks out of the side of your head (called the pinna), continues through the ear hole and down a small tunnel called the ear canal until it reaches the eardrum 
.
The pinna acts like a funnel, catching sounds as they whizz past your head, enhancing them, and directing them down into your ear canal. At the end of the ear canal the sound hits the eardrum and vibrates it.
The Middle Ear
The other side of the ear drum (shown left) is attached to a tiny set of three bones 
called the hammer, the anvil and the stirrup (also known by their Latin names of malleus, incus, stapes).
As the eardrum vibrates, the three bones move backwards and forwards in time with the vibrations, causing the stirrup to vibrate a second, smaller 'eardrum' called the oval window. When the sound has finished coming through the middle ear, it is about 22x louder then when the sound came in – and it's all due to these tiny bones and their relationship to the eardrum and oval window.
The Inner Ear
On the other side of the oval window is a salty liquid in a long, curled up container called the cochlea 
. Because the oval window is being pushed, it forces the liquid away from itself in waves along the length of the cochlea, a bit like the way you can create waves in a bath by sliding backwards and forwards.
These waves will ‘peak’ (i.e. the crest of the wave) at specific places along the cochlea based on how high-pitched or low-pitched the sound is, triggering a nerve message that tells the brain whereabouts along the cochlea the wave peaked and how strong the peak was. The brain interprets these nerve messages as sound.
Summary
You will be able to see from this brief explanation of how our ear works that there are three separate sections within the ear, each playing a part in getting the sound up to our brains.
So if something goes wrong in one of these sections, sound will not reach the brain as well as it should. This is what is commonly termed a “hearing loss”.