There are different kinds of lightning: within the clouds, between the clouds, from clouds to clear air, and from cloud to ground.
The lightning that concerns people the most is cloud to ground. These flashes start fires, splinter trees, knock out electrical power and can even kill people. It is estimated that cloud to ground lightning strikes represent only about 20 percent of all lightning strikes.
Cloud to ground lightning occues when negatively charged electrons shoot downward in a forked pattern, known as a stepped leader. As the stepped leader nears the ground, it draws a positive charge, usually through something high such as a tree or building. As the leader and the streamer come together, a powerful electrical current begins flowing and contact begins the return stroke with an intense wave of positive charges traveling upward about 60,000 miles per second. This is the light that we see and the process can repeat several times along the same path in less than half a second, making lightning flicker.