The term heart attack refers to damage to the muscle of
the heart, usually from a lack of blood flow. Most of the time a blood
clot forms in one of the arteries that supply heart muscle with blood,
blocking the flow of blood. As the heart muscle starves it begins to
die, causing chest pain and other heart attack symptoms.

Cardiac arrest happens when the heart stops pumping blood. With
no pumping action, the blood pressure falls and blood is no longer
forced into the muscles and organs of the body. There are all sorts of
reasons the heart will stop pumping blood. It might get extremely weak
or begin quivering uselessly. It might also start racing so fast it can
no longer fill with blood, which causes the blood pressure to fall
dangerously.