@shlary artherosclerosis is the narrowing of blood vessels normally due to history of high cholesterol which builds up, inside the walls, eventually they may block the flow of blood. An Aneurysm is totally different, it is a localized bulge of the artery, think of a small balloon that happens on a weak wall spot. Aneurysms can happen almost in any artery, an AAA specifically happens on the aorta in the section that travels through the abdomen (aneurysms can happen on any spot of the aorta and are named differently depending on the section of the aorta) , as they grow larger they can rupture or dissect and those are critical events, usually fatal.