What is the difference between a brain aneurysm and an avm?

Good question: Aneurysms are weakend areas of blood vessels that can rupture causing hemorrhage and even death. Avm's are twisting of blood vessels in the brain when arteries communicate directly with veins of the brain with intervening brain tissue. They cause increase blood flow through the weak walled veins causing them to become engorged and large. Avm's can also have aneurysms form on them.

Pathology differs: Brain aneurysms occur at the junction of blood vessels ususally at the base of the skull. They appear like a ballooning or sac that can rupture. When this happens this leads to subarachnoid hemorrhage that can be devastating. Avms are about 1/10 as common. These are a "tangle of vessels" where blood from the arteries goes directly into the veins. These also bleed leading to neurological problems.