Looked up dementia but still unsure what it is.?

See below. It is a loss of mental function thay affects daily life. It involves memory loss, decreased activity and change of personality. Dementia is the broad term. Alzheimer's disease, vascular dementia and lewy bodies dementia are the specific type of dementia along with at least 50 other specific type of dementia.
Def. Dementia. This is one definition of dementia. Definition:dementia is a loss of mental ability severe enough to interfere with normal activities of daily living, lasting more than six months, not present since birth, and not associated with a loss or alteration of consciousness. Basically is the gradual loss of the intellectual functions or intelligence with significant difficulty to remember recent events.
Dementia. Is a syndrome of cognitive decline that effects a person's ability to function. Simple forgetfullness does not interfer with functioning. Hope this helps.