Dementia involves symptoms of loss of brain function, including memory, as well as distortions in personality that are out of character. Patients may also lose the ability to perform basic motor functions like tying shows or getting dressed. It is different than delirium in that it is, for the most part, in most ways, permanent. While dementia is usually associated with the elderly, it is no longer considered a normal part of the aging process.
Delirium, on the other hand, develops quickly in most patients, and can involve hallucinations, and the patient may well be disoriented. It hits recent surgical patients, those using hard drugs and the elderly more often whereas dementia can develop in any setting. Delirium is often temporary, where dementia is not. Motor skills are usually maintained, while the patient's connection to reality is not in the near term.
Amnesia is a condition which can arise at any age, for any number of reasons, typically though as the result of a stroke, or traumatic injury to the brain, or concussions. Motor skills are retained, but a classic symptom is being able to remember everything except new information. Another kind (retrograde amnesia) causes the person to lose all long term memory and can only operate in the present. They can still, however, understand all language, which is not true in dementia patients.