Alzheimer's Disease is the most common type of Dementia which you probably could have guessed as it is the a very widely used term now-a-days.
It is a physical disease which affects the brain. During the course of the disease, proteins build up in the brain to form structures called 'plaques' and 'tangles'. This leads to the loss of connections between nerve cells, and eventually to the death of nerve cells and loss of brain tissue. Alzheimer's is a progressive disease, which means, that gradually, over time, more parts of the brain are damaged. So over time, more symptoms develop and become more severe.
Alzheimer's disease mostly affects people over the age of 65. Once above this age, a person's risk of developing Alzheimer's disease doubles approximately every five years. One in six people over the age of 80 have a form of dementia.
More information found here.