Dementia is a decline in mental function that may interfere with the ability to perform daily activities. Dementia is not a disease. Instead it describes a collection of symptoms (most commonly memory loss, behavioral changes and problems with language) that generally occur together and can be caused by certain conditions. There are almost 7 million people with dementia in the United States, according to the National Institute of Neurological Disorders and Stroke (NINDS).

Dementia is classified in many different ways. Some physicians and scientists classify dementia as being either cortical or subcortical. Cortical dementia occurs as a result of impairment or damage to the cerebral cortex. This is the outer layer of the brain and is associated with memory and language, along with many other aspects of consciousness. Cortical dementias (e.g., Alzheimer's disease, Creutzfeldt-Jakob disease) often result in loss of memory and language skills.

Subcortical dementia results from impairment or damage to parts of the brain underneath the cerebral cortex. Because the cortex can be undamaged, people with subcortical dementia (such as those with Huntington’s disease) rarely experience memory loss and problems with language. Instead, people with subcortical dementia can experience behavior and personality changes, resulting in socially inappropriate and unusual actions.

Some forms of dementia are progressive, which means that they get worse over time. Alzheimer's disease, Lewy body dementia and frontotemporal dementia are all forms of progressive dementia. Most progressive forms of dementia are poorly understood by physicians and scientists and can be difficult or impossible to treat. Other forms of dementia, especially those caused by factors such as medical reactions, alcohol abuse and malnutrition, are usually temporary and reversible.