AED, or automated external defibrillator, are small electronic devices that were designed to allow minimally-trained people to provide lifesaving defibrillation (electric shock to the heart) to victims of sudden cardiac arrest.
Unlike the defibrillators seen on medical TV shows, AEDs are small, lightweight and very easy to operate. They are about the size of a lunch box and have adhesive electrode pads that rescuers attach to the person's chest.
An AED is very simple to use yet houses the same sophisticated defibrillation technology relied on by emergency medical services personnel and physicians.
An AED is programmed to tell rescuers exactly what to do using voice and visual prompts. Rescuers attach adhesive electrode pads to the person's chest. Through these electrodes, the AED is designed to automatically analyze the electrical activity of the heart to determine if a "shockable" rhythm is present. An AED is so easy to use even untrained school children can operate one quickly and correctly. 
With voice prompts and pictures the AED guides rescuers through the resuscitation process, advising when to give CPR. If the AED determines the person's heart needs a shock, it tells rescuers to stand back so a shock can be safely given through the adhesive electrode pads affixed to the person's chest. (Note: Some AED models will tell the user to push a button to shock and then ‘stand clear’ of the victim, while others are fully automatic and will automatically give the shock after giving rescuers a 'stand clear' warning.)
The delivery of an electrical shock to a heart experiencing SCA briefly stops all electrical activity in the heart. This brief break from the previous electrical chaos can be enough for the heart to restart with a normal rhythm.
Not everyone can be saved from SCA, even with defibrillation. But early defibrillation, especially when delivered within three to five minutes of a person's collapse from SCA, does provide the best chance for survival.
It's that simple.
 Gundry, J., et al. 1999. Comparison of naïve sixth-grade children with trained professionals in the use of an automated external defibrillator. Circulation 100: 1703-7.