Definition of Hearing loss
A full or partial decrease in the ability 
to detect and recognize sound in one or both ears.
Types of Hearing Loss
There are different types of hearing loss, depending on which part of the hearing pathway is affected. A specialist will always try to localize where in the hearing pathway the problem lays, so as to be able to classify the hearing loss as belonging to one of the following groups. This is most important in determining the appropriate treatment.
Conductive hearing loss usually involves a reduction in sound level, or the ability to hear faint sounds, this type of hearing loss can often be medically or surgically corrected.
In cases of conductive hearing loss, sound waves are not transmitted effectively to the inner ear because of some interference in:
The external ear canal
The mobility of the eardrum (problems with the mobility of the eardrum are often caused by accumulation of fluid in the Eustachian tube, the tube that connects the middle ear to the back of the throat)
The three tiny bones inside the middle ear
The middle-ear cavity
The openings into the inner ear
The Eustachian tube.
Examples of conditions that may cause a conductive hearing loss include:
Conditions associated with middle ear pathology such as fluid in the middle ear from colds, allergies (serous otitis media), poor Eustachian tube function, ear infection (otitis media), perforated eardrum, benign tumors
Impacted earwax (cerumen)
Infection in the ear canal (external otitis)
Presence of a foreign body
Absence or malformation of the outer ear, ear canal, or middle ear
Otosclerosis, a condition in which the ossicles of the middle ear harden and become less mobile.
Sensorineural hearing loss
Sensorineural hearing loss occurs when there is damage to the inner ear (cochlea) or to the nerve pathways from the inner ear (retrocochlear) to the brain
Sensorineural hearing loss not only involves a reduction in sound level, or ability to hear faint sounds, but also affects speech understanding, or ability to hear clearly.
Sensorineural hearing loss cannot be medically or surgically corrected.
A large variety of hearing impairments fall under this category.
Common causes of sensorineural hearing loss
Age-related hearing loss (presbyacusis): This is the natural decline in hearing that many people experience as they get older. It's partly due to the loss of hair cells in the cochlea.
Acoustic trauma (injury caused by loud noise) can damage hair cells.
Certain viral or bacterial infections such as mumps or meningitis can lead to loss of hair cells or other damage to the auditory nerve.
Ménière's disease, which causes dizziness, tinnitus, and hearing loss.
Certain drugs, such as some powerful antibiotics, can cause permanent hearing loss. At high doses, aspirin is thought to cause temporary tinnitus - a persistent ringing in the ears. The antimalarial drug quinine can also cause tinnitus, but it's not thought to cause permanent damage.
Acoustic neuroma. This is a benign (non-cancerous) tumor affecting the auditory nerve. It needs to be observed and is sometimes treated with surgery.
Other neurological (affecting the brain or nervous system) conditions such as multiple sclerosis, stroke, or a brain tumor.
Mixed Hearing loss
Mixed hearing loss is a combination of conductive and sensorineural hearing loss, in this type damage may be in the outer or middle ear and in the inner ear (cochlea) or auditory nerve.
Central Hearing Loss
In central hearing loss, the problem lies in the central nervous system, at some point within the brain. Interpreting speech is a complex task. Some people can hear perfectly well but have trouble interpreting or understanding what is being said.
Basically, the problem involves a person's inability to filter out competing auditory signals. People with central auditory processing disorders have difficulties that include:
Problems "hearing" when there are several conversations going on
Inability to read or study with the radio or television on
Problems reading if someone turns on a vacuum cleaner or air conditioner near them
Generally missing the first sentence from people talking to them if they are involved in an auditory attention task (such as watching television).
There is no good treatment for central auditory processing disorders other than educating the person, family, and friends, and trying to control the environment.