In a nutshell, dementia is a symptom, and Alzheimer's disease is the cause of the symptom.
When someone is told they have dementia, it means that they have significant memory problems as well as other cognitive difficulties, and that these problems are severe enough to get in the way of daily living.
Alzheimer's disease is a physical illness that causes radical changes in the brain. As healthy brain tissues degenerate persons suffering from Alzheimer's experience a steady decline in memory and the ability to use their brain to perform tasks.
Dementia is the gradual deterioration of mental functioning, such as concentration, memory, and judgment, which affects a person’s ability to perform normal daily activities.
Dementia is a an illness that usually occurs slowly over time, and usually includes a progressive state of deterioration. The earliest signs of dementia are usually memory problems, confusion, and changes in the way a person behaves and communicates.
Original content Bob DeMarco, the Alzheimer's Reading Room