A heart attack happens when blood flow to part of the heart muscle is blocked, usually by a clot in a narrowed artery. The person is typically conscious and has symptoms such as chest pain, breathlessness and sweating. It is a circulation problem.
Cardiac arrest is an electrical problem: the heart's rhythm fails and it suddenly stops pumping. The person collapses, becomes unresponsive and stops breathing normally. A heart attack can trigger a cardiac arrest, but they are not the same thing.
Both are emergencies. Call your local emergency number immediately. For a heart attack, keep the person calm and seated. For cardiac arrest, start CPR straight away and use an automated external defibrillator (AED) if one is available, while waiting for help. Knowing CPR can save a life in the minutes before paramedics arrive.