CABG (coronary artery bypass graft) surgery uses healthy blood vessels to bypass diseased coronary arteries. Since it's such an invasive procedure, usually the surgeon will bypass any arteries that would cause trouble in the future.
The first "bypass" is the LIMA (left internal mammary artery), which is typically grafted to the LAD (left anterior descending [coronary] artery), distal to the occlusions. Portions of saphenous veins are usually used next, to jump over diseased parts of other coronary arteries.
During the procedure, another kind of "cardiac bypass" is used. The heart is stopped (so the surgeon can sew blood vessels), and a heart-lung machine is hooked up that oxygenates and pumps blood. Catheters are placed in the superior and inferior venae cavae and the ascending aorta to effectively bypass the heart and lungs during surgery.
The best experience I've had in medical school is when I got to perform a median sternotomy... cardiac surgery is awesome.
Cheers,
doepug