Atherosclerosis is a condition in which a material called plaque builds up on the inner walls of the arteries. This can happen in any artery, including the coronary arteries. The coronary arteries carry oxygen-rich blood to your heart. When atherosclerosis affects the coronary arteries, the condition is called coronary artery disease (CAD).
Development of atherosclerosis (from: Greenland P, Gidding SS, Tracy RP. Commentary: lifelong prevention of atherosclerosis: the critical importance of major risk factor exposures. Int J Epidemiol 2002; 31:1129-34):
An atherosclerotic plaque obstructs blood flow as shown in this picture (from: http://www.nhlbi.nih.gov/health/dci/Diseases/Atherosclerosis/Atherosclerosis_WhatIs.html):