We are asked this question a lot, many people think heart attack and cardiac arrest are the same when in reality they're very different.
While heart attack can lead into a cardiac arrest the starting point for the two are vastly different.
Heart attack is a circulation problem where the blood can't get around the body as easily because a blood clot or narrowed artery is stopping the heart from working effectively. As often the blood can continue to travel around the body to some extent, people can remain conscious and continue to breathe though this will become difficult and often people will experience symptoms such as dizziness and feelings of faintness among other things.
You can read more about heart attacks here
Cardiac arrest is an electrical problem. Your heart works on electrical impulses to make it beat in the correct order to move the blood around the body. In the case of a cardiac arrest the electrical impulses have come out of sync or have stopped completely. When this happens no oxygen can get around your body to your brain and you will become unconscious and stop breathing very quickly.
In either of these situations you should call 999 for an ambulance if they are breathing and awake sit them in the W position and take advice from ambulance control.
If they are not breathing start CPR