To understand lightning I had to learn about the nature of electricity: what
                         it is, and how it moves. I found out that everything in the world is made of
                         electricity and that electrical forces are responsible for holding things
                         together--and sometimes for pushing them apart.
Everything is made of atoms that are, in turn, made out of charged
                         particles. All charged particles come in one of two types: positive and
                         negative (or plus and minus). The minus particles are the electrons, and
                         the plus particles are the much heavier protons which are buried deep in
                         the nucleus. (There are also heavy neutral particles in the nucleus called
                         neutrons, but they really don't enter into our story.)
                         The awesome power of the lightning stroke originates in the thunderstorm
                         cloud where charges somehow become separated. There are several
                         complicated theories that try to explain the actual mechanism of this
                         charge separation, but no one really knows what pulls the charges apart
                         in a thunderstorm cloud. It is believed that somehow water drops in the
                         cloud become negatively charged and, being heavier than the surrounding
                         air, fall to the bottom of the cloud. Meanwhile, the positive ions left behind
                         are swept upward to the top of the cloud by the warm updrafts within the
                         thunderhead. As more and more charges separate, parts of the cloud
                         become so highly charged that the electrical forces tear nearby air
                         molecules apart, making more charged fragments.
In a thunderstorm, the safest place to be is inside a large building
                         equipped with lightning rods. A vehicle such as a car provides complete
                         protection by surrounding you with metal, which will conduct the lightning's
                         charge safely to the ground. But it you are caught outside, don't stand
                         under a tree. The tree acts just like a lightning rod and if you become part
                         of the conducting path to the ground, it's good-bye you. Even if the current
                         from the lightning stroke doesn't hurt or kill you, the tree might. When the
                         current of a lightning bolt passes through a tree, the sappy interior can be
                         heated to the boiling point, and the tree can explode! When lightning
                         strikes the ground, the charges flow outward along the ground. If you are
                         standing nearby with your feet apart, the current will flow up one leg and
                         down the other, possibly killing you. Many cattle are lost to lightning
                         because they can't keep their feet together. If they did, they'd lose their
                         balance.
So, now I know a few things about lightning. But the thought of all those
                         charges rushing to and fro, although interesting, does not make the
                         spectacle less impressive or less frightening. Now I sit up and watch the
                         light show, but still feel that urge to dive for the covers.

This information came from the website: http://www.exploratorium.edu/ronh/weather/weather.html