Understanding of disability as such often presents itself as one of two distince (though sometimes overlapping) approaches.
The Medical Model understands a disability as a physical or mental impairment of the individual and its personal and social consequences. It regards the limitations faced by people with disabilities as resulting primarily, or solely, from their impairments. This pushes people with disabilities into the passive role of patients. The aim of a medical approach is to make people with disabilities “normal” – which of course implies that people with disabilities are in some way abnormal. The issue of disability is limited to the individual in question: in case of disability, the disabled person has to be changed, not society or the surrounding environment.
The Social Model, in contrast, understands disability as a relation between an individual and the social environment: the exclusion of people with certain physical and mental characteristics from major domains of social life. Their exclusion is manifested not only in deliberate segregation, but in a built environment and organized social activity that preclude or restrict the participation of people seen or labeled as having disabilities.
Shortcomings in the way society is organized mean that people with disabilities face specific types of discrimination and barriers to participation, including attitudinal (fear, ignorance, and low expectations influenced by culture and religion), environmental (physical inaccessibility affecting all aspects of life), and institutional (legal discrimination denying rights to marry, parent, attend schools, etc.)
Adapted from "Disability: Definitions, Models, Experience," from The Stanford Encyclopedia of Philosophy: http://plato.stanford.edu.
See the Disabled World list of models for additional definitions and discussion: http://www.disabled-world.com