Disability is often distinguished by the interaction between individuals and their environment. Individuals with similar conditions can differ in the severity of their impairment and their ability to meet normal role expectations. Some will have family situations that enable them to overcome social barriers. Others will use equipment to overcome physical barriers. Thus, functioning and disability can be viewed as a complex interaction between the health condition of the individual and the contextual factors of the environment they live in.

To more clearly define this view, the World Health Assembly approved in 2001 the International Classification of Functioning, Disability and Health and its abbreviation of "ICF." This classification is the revision to the International Classification of Impairments, Disabilities, and Handicaps.

The ICF is structured around the following broad components:
(1) Body functions and structure, (2) Activities (related to tasks and actions by an individual) and participation (involvement in a life situation), and (3) Additional information on severity and environmental factors

Sources:

http://www.who.int/classification/icf/intros/ICF-Eng-Intro.pdf

http://www.cdc.gov/nchs/about/otheract/icd9/icfhome.htm