Dementia is not a specific disease. It's a term that describes a wide range of symptoms associated with a decline in memory or other thinking skills. It is called dementia if the decline is severe enough to reduce a person's ability to perform everyday activities.
Alzheimer's disease accounts for 60 to 80 percent of dementia cases. Vascular dementia, which occurs after a stroke, is the second most common dementia type. There are many other conditions that can cause symptoms of dementia. Some of these are reversible, such as thyroid problems and vitamin deficiencies.
Further information about types of dementia from the Alzheimer's Society.