According to the ADA Tool Kit , "a physical impairment is a physiological disorder or condition, cosmetic disfigurement or anatomical loss impacting one or more body systems. Examples of body systems include neurological, musculoskeletal (the system of muscles and bones), respiratory, cardiovascular, digestive, lymphatic and endocrine." It goes on to say a "mental impairment is a mental or psychological disorder. Examples include mental retardation, emotional or mental illness, and organic brain syndrome."
For more information about disability-related terms, consult Glossary of Disability-Related Terms. 
-  ADA Tool Kit
-  Glossary of Disability-Related Terms.