Various mechanisms create a stratified charge in a storm cloud. The charge on the base of the cloud induces an opposite charge on the surface of the earth beneath it. (Remember playing with magnets as a kid? Like charges repel and opposite charges attract.) As the storm cloud builds, it increases the potential difference between the cloud base charge and the ground charge, with the cloud base charge trying to pull the ground charge off the surface of the earth.
As the charged storm cloud travels through the atmosphere, it drags its ground charge along beneath it. When the ground charge reaches a structure, the attraction of the cloud charge pulls it up onto the structure, and concentrates the ground charge on the structure. If, before it moves away, the charge on the cloud base manages to concentrate enough ground charge potential on and around the structure beneath it to overcome the dielectric of the intervening air, an arc, or lightning strike, occurs.
When the dielectric of the air is overcome and lightning is going to strike, the process begins with the formation of stepped leaders branching down from the cloud. These stepped leaders propagate in jumps of about one hundred and fifty feet and downward towards the ground. Stepped leaders are the tendril-like branches extending down from the cloud which are visible in a photograph of a lightning strike. We see a lightning strike in two dimensions, but the field of stepped leaders is three dimensional, it has depth too.
When the stepped leaders reach within five hundred feet or so of the ground, the electric field intensity on the ground becomes so strong that objects and structures on the ground begin to break down electrically and respond by shooting off streamers of ground charge upward toward the stepped leaders. When a streamer connects with a stepped leader, the ionized path becomes the channel for the main lightning discharge. The other streamers and stepped leaders never mature.