As far as I know (my wife is an emergency physician), despite what the name suggest, CPR is not really for "bringing back". Is a technique to keep the blood flowing so the brain doesn't suffer damage until other techniques are applied. Also, as she explained to me, the defibrillator is only used on certain kind of arrests, it can actually make things worse on others.
CPR circulates blood to the oxygen and brain, keeping it going long enough for the heart to (maybe) be restarted.
For the first few minutes after someone's heart stops beating, it probably hasn't actually stopped, it is just beating in a chaotic way that isn't actually moving any blood (this is known as "ventricular fibrillation, hence the "defibrillator"). The electrical discharge of the defibrillator is like someone standing up in a loud room and blowing an air horn. It shuts everyone up for a second, and hopefully the heart's normal "pacemaker" can take back over.
There are some medications that can be given to "strengthen" that pacemaker, and to make the rest of the heart a bit less "twitchy" (and therefore less likely to go back into a lethal arrhythmia).
There is a related arrhythmia known as ventricular tachycardia where the heart is beating too quickly to move any blood (and the beating is being triggered by random cells in the heart, not be the heart's intrinsic pacemaker).
If someone is unresponsive and doesn't appear to be breathing, high quality CPR and early defibrillation are what will help them. If you have an AED, put it on them and follow the prompts it gives. And AED will not do anything that would make the patient's condition worse (to hurt someone with a defibrillator, you need to be using a manually controlled one).
