Which part of the brain does Alzheimer's affect?

Alzheimer's Disease affects the cognitive, or thought, processes of the brain. Alzheimer's is characterized by two abnormal structures in the brain: amyloid plaques and neurofibrillary tangles. Although the distribution varies among individuals, the areas commonly affected in the brain include the association cortical and limbic regions. Well to answer your question more accurately, it affects the brain in essence meaning every aspect of it. So Alzheimer's affects all parts of your brain because of the degeneration.
Alzheimer's Disease causes extreme shrinkage of the Cerebral Cortex, certain subcortical regions and the Hippocampus due to the loss of neurons and synapses.
brain regions responsible for learning, reasoning, and memory. Autopsies of persons with AD show that these regions of the brain become clogged with two abnormal structures--neurofibrillary tangles and senile plaques.
The areas most affected are in the temporal lobe. The cerebral cortex and hippocampus tend to shrink, and the brain ventricles increase in size.