Impairment, disorder, and disability are terms introduced by the World Health Organization (WHO) in 1980. In the WHO classifications, impairment is defined as an abnormality of a structure or function (e.g., an abnormality of the ear or auditory system). Disability is the functional consequence of impairment (e.g., inability to hear certain sounds or inability to speak clearly). Handicap is the social consequence of impairment (e.g., isolation, loss of job, or having to make career changes as a result of communication difficulties). By applying the WHO definitions, distinctions are made that apply to how one functions as a result of an impairment. Not all impairments result in disabilities. One may also be disabled but not handicapped.

I highly recommend the following article, from which this information was taken. Source: American Speech, Language, Hearing Association