Dementia is any disease of the brain which reduces ones ability to think. It can be caused by a variety of factors, even things like arterial hardening and reduced oxygen flow in the brain.

Alzheimer's disease is one such cause of dementia (and a very prominent one) with very specific causes (some of which I mentioned above in the post!). So in short, all Alzheimer's is dementia, but not all dementia is Alzheimer's.