Excellent question. When an individual in considered deaf it means that they have total loss or partial loss of the ability to detect sound. This can be caused by many things including injury to the ear, aging, disease and sometimes deafness is inherited in your genes.In most cases the physical structure of the ear is changed in a way that does not allow the ear to work properly. The parts of the ear that are affected most often are the ear drum and the ear bones. The ear has three main parts, the outer ear, the middle ear and the inner ear. The eardrum is in-between the outer and middle ear. The ear bones are in the middle ear. When a sound it made it travels through the air in a wave that vibrates the eardrum. The vibrating eardrum transfers the sound vibrations to the bones that can vibrate to transfer the sound to the nerves that go to the brain. When the nerves receive the vibrations they send this information to the brain using electrical pulses. Those pulses are interpreted in your brain as sound (music, birds, your mom telling you to do your homework, etc.). Take a look at the picture here
ear
to see where the eardrum, ear bones (incus, malleus and stapes) and the auditory nerve to the brain are located. Now that you understand how the ear "hears" sound it makes sense that when the eardrum, ear bones or nerves are broken they can not properly detect sound.

Keep up the curiosity and don't forget to listen.

Great question!The ear is a complex arrangement of many moving parts and nerve cells, and a breakdown in any part of the ear can lead to deafness. There are two main types of deafness. The first is conductive deafness which comes from interference with the transmission of sound waves through either the outer or the middle ear. The most common form of conductive deafness is caused by a buildup of earwax (cerumen) that obstructs the ear canal and prevents sound waves from reaching the inner ear. The second type of deafness is Sensorineural, or nerve, deafness which arises from the inability of nerve impulses to reach the auditory center of the brain because of nerve damage either to the inner ear or to the brain. The most common cause of sensorineural deafness is disease (chicken pox, influenza, and others). Hope this answers your question. Keep on asking!