Audiology is the branch of Science that studies hearing, balance and related disorders. Its practitioners, who treat those with hearing loss and proactively prevent related damage, are audiologists.
Employing various testing strategies, audiology aims to determine whether someone can hear within the normal range, and if not, which portions of hearing (high, middle, or low frequencies) are affected and to what degree. If an audiologist determines that a hearing loss or vestibular abnormality is present he or she will provide recommendations to a patient as to what options (e.g. hearing aids, cochlear implants, surgery, appropriate medical referrals) may be of assistance.
In addition to testing hearing, audiologists can also work with a wide range of clientele in rehabilitation (cochlear implants and/or hearing aids), pediatric populations and assessment of the vestibular system.
See Wikipedia, Audiology.