Dementia describes a group of symptoms such as loss of memory, judgment, language, complex motor skills, and other intellectual function, caused by the permanent damage or death of the brain's nerve cells. It is an umbrella term that covers all types of dementia.
Alzheimer's is a type of dementia. There are many types of dementia including Vascular Dementia, Lewy Body Dementia, Frontotemperal Dementia, Creutzfeldt-Jacob (Mad Cow) Dementia, Normal Pressure Hydrocephalus (NPH), Huntington's Disease, Wernicke-Korsakoff Syndrome, Mild Cognitive Impairment (MCI), and mixed dementia. Some such as Normal Pressure Hydrocephalus (NPH) and Wernicke-Korsakoff Syndrome can be improved with treatment. Others can not be improved with treatment. This is why it is important to determine which dementia is present.
To put it another way... Dementia is like Cancer. Alzheimer's is the equivalent of having Lung Cancer, Colon Cancer, Breast Cancer, or another type of cancer.
Hope that helps