When a person has a heart attack, immediate care is vital. A heart attack victim will only survive for a few minutes without help.
So let's start at the beginning - What is a heart attack? This video offers a nice explanation:
Where do the fatty deposits come from, and what do they look like? This video is gross, but offers a graphic demonstration:
So, the heart stops pumping blood. Now what? The best way currently available to restart a normal heart rhythm is known as the heart defibrillator. Here is the classic scene (complete with dramatic music) that we see on TV when the defibrillator comes out:
Now there are also smaller, automated defibrillators available in offices, airports, amusement parks, schools, etc. This video shows how a typical unit is used:
The heart of a heart defibrillator is a big capacitor. A battery takes perhaps half a minute to charge this capacitor, in the same way that a camera flash charges a capacitor. In this video you can see a typical capacitor from a defibrillator (it's the large item on the right with the red wires), rated at 54 microfarads and 4,200 volts.
Once charged up, this capacitor contains a huge amount of energy (in this case 970 joules). The defibrillator delivers that charge in a millisecond through the paddles, and the hope is that the shock resets the heart so it can begin beating normally.
For more info see: How the heart works.