The exact science of why lightning occurs in still not secure, as it is a very difficult and dangerous phenomenon to monitor and measure. What we do know, however, is that it occurs because of an imbalance between electrical charges. Just like a magnet, different charges (positive and negative) attract each other. These different charges occur naturally in both the ground and clouds, due to the complex working of the water cycle. Lightning happens when these charges are opposite and begin to attract each other. The air between the two charges tries keeps them separate, but eventually breaks down allowing an electrical discharge, the lightning, to travel between the two areas. Different types of lightning are classified depending on where the strike travels from and too, and the electrical charge of these start and end points. There are four main types of lightning that occur, and several different forms these strikes can take. Once you understand how lightning works, it is easier to see why it is classified in the way that it is.
1. Cloud-to-Ground Lightning
This is the classic cartoon lightning we see on films and TV, the one that strikes the tree, or a person making their skeleton glow. Cloud-to-ground lightning is exactly how it sounds; the lightening strike travels from the bottom of a cloud, to the ground. In this instance the lightning strike travels from the negatively charged bottom o the cloud to the positively charged ground. This type of lightning, strikes the highest object (a tall building, or your umbrella if you’re in a field). Only 20 percent of all lightning flashes ever reach the ground, so this may be a classic on TV but is not that common in reality.
2. Ground-to-Cloud Lightning
The opposite of the first type of lightening, this is when the strike travels from the ground up to the cloud. The strike moves from the negatively charged ground to the positively charged centre of the cloud above.
3. Intracloud Lightning
This is the most common type of lightning, and it occurs when there are positive and negative charges within the same cloud. This all occurs within the cloud and often appears as a bright flash, rather than a streak of lightning in the sky.
4. Intercloud Lightning
This is similar to Intracloud lightning but is much less common. This occurs when the charges in two neighbouring clouds are different and the strike travels through the air between the clouds.
For further information:
http://wvlightning.com/types.shtml - A comprehensive guide with excellent illustrations
http://science.howstuffworks.com/lightning2.htm - A very precise guide to the formation of lightning, beginning with the water cycle
http://www.centennialofflight.gov/2003FF/lightning/types.html - A clear guide to the types and forms of lightning.