Lightning is a natural form of electricity. Lighting occurs due to an accumulation of charge, electrons if you like, in the air. As a storm grows, electrical charges build up in the clouds. At the same time, oppositely charged particles are growing in number on the earth's surface. As you know opposite charges attract and due to the large number of charges this attraction grows quickly. At some point the attraction becomes large enough to overcome air's resistance to electrical flow, in this case the flow of charges between the ground and the clouds. These particles move toward each other at incredible speeds and when they meet they complete an electrical circuit.

Charge from the ground then surges upward at nearly one-third the speed of light and we see a bright flash of lightning. So, lightning is basically just the result of static electricity.

Thunderstorms build up because of something called "convection". You probably know that hot air rises and cool air falls. When the sun heats the air near the ground, it tends to rise, but the higher it goes the more it cools off, which eventually causes it to fall back down again. This circulation is called convection. If the air carries with it moisture, you form thunderclouds. The exact mechanism of how and why lightning occurs isn't really well understood. Here's one theory: as the convection process goes on, warmer water droplets going up bump into cold ice crystals going down. Just like rubbing your feet on the floor builds up a static charge on your body, so do these collisions between the water drops and ice crystals build up a static charge in the cloud, which eventually makes a spark - just a really big one. But this is only one theory.