Your HEART is the central pump of your body.  It pushes blood to even the most faraway tips of your toes in order to bring oxygen to all the cells that make up your body.  The blood is brought to the heart through tubes called arteries.  The arteries break down into smaller tubes called veins and capillaries in order to get to those hard-to-reach places. 

The blockages that cut off blood in the heart are usually caused by clots or a buildup of plaque [Excess cholesterol, calcium, fat and other substances which float in your blood] along the walls of the arteries.  When plaque builds up inside arteries, it is called atherosclerosis. 

When plaque builds up in an artery, it often forms a thin skin to hold it together.  If this layer ruptures, the body will try to heal it by forming a blood clot over it.  This clot is what blocks the flow of blood to the heart and causes the heart attack. 

In rare cases, arteries spasm and contract-cutting off the flow of blood and causing a heart attack.