Alzheimer's disease is named after Dr. Alois Alzheimer, a German doctor who first described this disease early in the 20th century.
it is the commonest cause of dementia in Western (developed) societies. probably is too for less developed countries but statistics are not as good there.
development of neurofibrillary tangles and neuritic plaques in the brain cause the symptoms of Alzheimer's and a definite diagnosis of AD is only really available on autopsy.
the disease course:
short term memory usually suffers first, followed by language skills, then judgement and emotional control later. progress is usually slow but can sometimes (rarely) be rapid. most patients with AD live for many years after being diagnosed with this disease.
a good writeup on this: Senile dementia/Alzheimer's type
read all about it: http://www.alzheimers.org/unravel.html