Image Taken
From: http://www.wrh.noaa.gov/Eureka/spotters/ nco/
Cloud to ground lightning originates
near the base of the cloud, and is initiated by a small discharge that releases
free electrons that move toward the ground forming a stepped leader. The stepped leader is an invisible
discharge, moving downward in discrete microsecond steps that are about 50
meters (165 feet) long. An upward
moving leader begins from the ground to meet the approaching downward moving
negative stepped leader once it is within 100 meters (330 feet) or less from
the ground. This upward moving leader
generally comes from objects that are taller than their surroundings, such as
trees or buildings.
Image Taken From: http://www.boltlightningprotection.com/lightning_physics.htm
After the
leaders have made contact, the visible lightning strike, called the return 
stroke, moves up from the
ground along the stepped leader’s ionized path. Multiple successive strokes occur along the original path,
separated only by several tens of milliseconds, continuing until the charge on
the lower part of the cloud is removed.
The entire process typically takes about half a second to complete, and
is referred to as a flash. The flickering effect that is often seen occurs
because the human eye is just able to discern the individual pulses of
luminosity created by each stroke.
Image
Taken From: http://www.dairyland.com/