Atherosclerosis is disease of the arteries when you get thickening and hardening of the vessels causing obstruction to the flow of blood. If it is in the arteries of the brain you can get stroke, dementia and death
Atheroscleosis refers to a build up of plaque that appears on the walls of the arteries. This also causes the arteries to harden.
Atherosclerosis is the build up of a waxy plaque on the inside of blood vessels.
(sources from www.answers.com)
Harding of the arteries cause by a collection of fatty cholesterol like deposits.
atherosclerosis itself is a medical term for sclerosis of the vessel (it can be arteiosclerosis, arteriolosclerosis etc.)