Dementia itself is a symptom of a variety of diseases and disorders. A general definition of dementia is a significant, generalized loss of intellectual abilities severe enough to interfere with social or occupational functioning. For a clinical diagnosis, memory impairment must be present in addition to other forms of cognitive impairment.

The following definition, where dementia is more plainly described, was found at Medline Plus (http://www.nlm.nih.gov/medlineplus/dementia.html):
 

Dementia is a word for a group of symptoms caused by disorders that affect the brain. It is not a specific disease. People with dementia may not be able to think well enough to do normal activities, such as getting dressed or eating. They may lose their ability to solve problems or control their emotions. Their personalities may change. They may become agitated or see things that are not there.

Memory loss is a common symptom of dementia. However, memory loss by itself does not mean you have dementia. People with dementia have serious problems with two or more brain functions, such as memory and language.

Many different diseases can cause dementia, including Alzheimer's disease and stroke. Drugs are available to treat some of these diseases. While these drugs cannot cure dementia or repair brain damage, they may improve symptoms or slow down the disease.

- National Institute of Neurological Disorders and Stroke
 

Generally, dementia is thought of as a progressive disorder, where the symptoms get worse over time. This is compared to other types of brain injury that are more static over time, such as a traumatic brain injury.

Alzheimer’s disease is the most well-known form of dementia, but dementia is a symptom of a variety of other disorders and diseases. If you have a form of dementia other than Alzheimer’s, you should find that much of the information included in Alzheimer’s resources is generic enough to be helpful for all types of dementia . But, also check out information targeted to your specific disorder/disease.

The following links provide resources for information about dementia symptoms and related diseases:

MedlinePlus: Health Topics/Dementia
http://www.nlm.nih.gov/medlineplus/dementia.html

ALZBRAIN.ORG: Information/resources about Alzheimer’s and dementia
http://www.alzbrain.org/

Alzheimer’s Disease International: Early symptoms
http://www.alz.co.uk/alzheimers/symptoms.html

Alzheimer’s Association: Related Dementias
http://www.alz.org/alzheimers_disease_related_diseases.asp