Dementia Is a Group of Symptoms:

Dementia isn’t a disease. It’s a group of symptoms that affect mental tasks like memory and reasoning. Dementia can be caused by a variety of conditions, the most common of which is Alzheimer’s disease.

As dementia progresses, it can have a devastating impact on the ability to function independently. It’s a major cause of disability for older people, and places an emotional and financial burden on families and caregivers.

The World Health Organization (WHO) says that 35.6 million people around the world are living with dementia.

Early symptoms of dementia can be mild and easily overlooked. It often begins with simple episodes of forgetfulness. People with dementia have trouble keeping track of time and tend to lose their way in familiar settings.

As dementia progresses, forgetfulness and confusion grow. It becomes harder to recall names and faces. Personal care becomes a problem. Obvious signs of dementia include repetitious questioning, inadequate hygiene, and poor decision-making.

In the most advanced stage, dementia patients become unable to care for themselves. Time, place, and people become more confusing. Behavior continues to change and can turn into depression and aggression.

So dementia is a group of symptoms that affect memory and reasoning.

Many forms of dementia can be treated with medication.

Dementia is a problem of the brain that you’re more likely to develop as you age:

Many conditions can cause dementia, including degenerative diseases like Alzheimer’s, Parkinson’s, and Huntington’s. According to the Cleveland Clinic, Alzheimer’s disease is responsible for 50 to 70 percent of all cases of dementia.

Infections such as HIV can trigger dementia. So can vascular diseases and stroke. Depression and chronic drug use are other possible causes. 

So Alzheimer's can cause dementia.

Alzheimer’s Is a Disease:

Alzheimer’s is a progressive disease of the brain that slowly impairs memory and cognitive function. The exact cause is unknown and there is no cure.

The National Institutes of Health (NIH) estimate that more than five million people in the United States have Alzheimer’s disease. Although younger people can (and do) get Alzheimer’s, symptoms generally begin after age 60.

The time from diagnosis to death can be as little as three years in people over 80 years old. However, it can be much longer for younger people. 

Damage to the brain begins years before symptoms show. Abnormal protein deposits form plaques and tangles in the brain of someone with Alzheimer’s disease. Connections between cells are lost and they begin to die. In advanced cases, the brain shows significant shrinkage.

It’s impossible to diagnose Alzheimer’s with 100 percent accuracy while a person is alive. The diagnosis can only be confirmed during an autopsy, when the brain is examined under a microscope. However, specialists are able to make the correct diagnosis up to 90 percent of the time, according to the NIH. 

See also what Alzheimer's does to the brain:

http://www.healthline.com/health-slideshow/alzheimers-and-brain

Someone explained to me recently a simple test to tell if someone has Alzheimer's or is just having normal forgetfulness.  Say someone can not remember the name for "spoon", but they can tell you what it is used for, ie you use it to bring foods to your mouth, this is normal aging forgetfullness.  A person with Alzheimer's will not know the word, nor what the items function is.

Also this new test should help, if in time it proves out ;)