Alzheimer's disease is a type of dementia and probably the most common type. Alzheimer's is characterized by profound memory loss like forgetting the names of children. Dementia is sort of a catch all phrase used by the health care establishment to describe the gradual loss of mental functioning. Signs and symptoms of dementia are language difficulties, failure to recognize or identify previously known objects, and decreased ability to think abstractly.
Dementia generally has an onset in the 60's or later. Alzheimer's disease can surface much younger like in the 40's. Another hallmark sign of Alzheimer's is urinary incontinence (rarely seen in dementia), and inappropriate mental affect ( laughing at something that should make you cry, and crying at something that should make to laugh).
Some authorities argue that the only true way to distinguish the two is at autopsy. The brains of Alzheimer's patients have anatomical and chemical variations ( high levels of aluminum ), that is not seen with demented patients.
The two diseases are quite similar. The relationship between Alzheimer's disease and dementia is that Alzheimer's disease is one possible cause of dementia. Because of this causal relationship it is quite common for the two conditions to be confused as the same.
In dementia a person's thinking and memory is affected to the point that a person begins to lose the ability to do things they were once able to do.
Determining whether the dementia is caused by Alzheimer's requires examination of the brain at the microscopic level, which at this present time in medical history is not an ability we possess. As such determining a patient's history, and abilities help point to the cause of dementia. Other types of tests like blood tests are also conducted.
Alzheimer's is a also a very slowly progressing disease that works backwards from a person's present day memory. It is almost like mentally ageing backwards while your body continues to age forwards.
Essentially the difference is that dementia is a symptom that causes an impairment to thinking and memory of many possible conditions, one of which is Alzheimer's.