When the negative charge in a cloud becomes great enough, it seeks an easy path to the positively charged ground below. The current looks for a good conductor of electricity, or a tall structure anchored to the ground (such as a tree or a tall building). The negative charge sends out a feeler, called a stepped leader, which is a series of invisible steps of negative charges.
As the stepped leader nears the ground, a positive streamer reaches up for it. Only then, once this channel is made, does the visible lightning happen. A return stroke runs from the ground to the clouds in a spectacular flash.
Though the bolt appears continuous, it is actually a series of short bursts. Most lightning strikes occur in less than a half second and the bolt is usually less than 2 inches in diameter.