Dementia is the general term used to refer to the impaired ability to remember, think, or make decisions that interfere with everyday activities. It is not a disease in itself, and does not refer to cognitive impairment alone.
Alzheimer's disease is the most common type of dementia, and involves the parts of the brain that control thought, memory, and language. It is a progressive disease that begins with mild memory loss and can progress to loss of speech and environmental response.