How is the brain affected by alzheimer's?

Multiple deficits. Alzheimer's disease is the most common type of dementia. Memory problems, word finding problems, word substitutions errors, getting lost in places that are familiar are often early symptoms. Also forgetting how to use tools that you used to know how to use. Finally, trouble evaluating info and following plans happen later.
Abnormal deposits. Deposits, called amyloid plaques and neurofibrillary tangles develop in a progressive fashion in parts of the brain affected.
Chemical loss. Alzheimer's is a multi-neurotransmitter deficiency syndrome, with damage to acetylcholine, epinephrine, serotonin, some glutamate pathways, etc. Therefore the deposits of misfolded protein do affect electochemical nerve to nerve connections.