Disability usually refers to difficulty carrying out tasks or activities of daily life. The patient can't take care of him or herself. For example, disability from a back injury might mean the person can no longer get dressed or bathe without help.
Impairment describes problems at the tissue level. Impairment is any loss of normal physical or mental abilities. Impairments are usually the result of disease, illness, or injury. Impairments occur at the level of tissue, or organs. Impairment from a back injury might cause a disc to rupture or ligament to tear.
Doctors and therapists use these terms to describe what patients can and can't do based on actual physical changes in the body.