CPR - Cardiopulmonary resuscitation is the process where external manual compressions move blood around the body to perfuse the organs, providing them with vital oxygen.
AED - Automatic External Defibrillator is the device that analyzes the heart in cardiac arrest, deciding whether a shock is required or not.
YOU are the key link in the chain of survival. As bystanders witnessing a cardiac arrest, your early intervention of starting CPR, sending somebody else to collect an AED and using these in combination will give the best chance of survival. Hhaving an AED with the patient quickly can increase the survivability by around 60% if there is a shock is requried.