The distribution of negative charge at the bottom of the cloud produces a potential difference on the order of 50 million volts between the earth and the cloud. Large potentials break down the air and there is a discharge. The movie below shows some detail of how the discharge occurs.

The process depicted in the movie is not well understood. People seem only to be able to describe different parts of the process. The pictures below identify two features.

At the begining of a lightning stroke, tendrils branch out from the the cloud. These tendrils are channels through which negative charges are attempting to flow. These are called stepped leaders.

When the stepped leader reaches the ground or some other object with a positive charge distribution there is a discharge. The discharge begins at the ground because it is the charges in the bottom of the leader that begin flowing first. So...we see an arc that starts at the ground and moves up to the cloud. This is called the return stroke. After the return stroke, other regions of charge in the cloud will try to use the channel establised by the the stepped leader. The result is that you may see several arcs as discharge occurs through the same channel.

Pictured below is a tree that is being struck lightning. To the left of the main strike there appears to be a tendril rising up from the tree. I think that this is called a streamer and originates from object at the that have a postive charge distribution.

If you're wonder what the doodle is all about , click on the doodle link.