The structure of lightning is caused by a series of events. This series begins when the bottom of a cloud becomes negatively charged inducing a positive charge on the earth's surface. As the strength of the electromagnetic field increases in strength the air molecules orient themselves so that the positive portion of the air molecule is toward the cloud and the negative portion is toward the earth. As the electromagnetic field continues to increase in strength the air molecules stretch and eventually rip apart and slam back together resulting in ionized air. This are ionizes in pockets.
Some pockets are more dense than others and therefore make better conductors.
There are several channels of ionized air which allow the lightning to propagate down this path which has less resistance. There are several branches which are racing for the ground at the same time. As the lightning nears the earth many positively charged leaders are sent up from the ground. This equilibration is called the return stroke. The return stroke accounts for the bright light associated with the lightning.