Dementia is a general term for selective degeneration of a particular part of the brain out of proportion to the normal, mild deterioration that happens with aging. The part of the brain that deteriorates determines the symptoms of the disease.
The most common form of dementia is Alzheimer's, which primarily affects memory. However, there are several other types of dementia that selectively affect other areas of the brain and cause slightly different symptoms. This makes proper evaluation and diagnosis critical.