What makes a jagged bolt of lightning suddenly leap across the sky?

In a fraction of a second, a jagged line of light rips across the sky, brighter than a thousand light bulbs and hotter than the surface of the Sun. It leaps about inside a cloud, or from cloud all the way down to the ground, in the blink of an eye. Something must gather quietly inside the storm first, then let go all at once. What builds up in there until it simply has to jump?