That's "lightning," not "lightening." Lightning is an abrupt electric discharge from cloud to cloud or from cloud to earth accompanied by the emission of light, while lightening is the process of making something lighter, or medically speaking, it refers to the sensation that a pregnant woman feels when the baby drops.
While lightning is one of the most beautiful displays in nature, it's also one of the most deadly natural phenomena known to man. Lightning is a sudden electrostatic discharge during an electrical storm between electrically charged regions of a cloud (called intra-cloud lightning), between that cloud and another cloud, or between a cloud and the ground.
All thunderstorms produce lightning and are very dangerous. If you hear the sound of thunder, then you are in danger from lightning. Lightning kills and injures more people each year than hurricanes or tornadoes; between 75 to 100 people.
Lightning is an electric current. Within a thundercloud many small bits of ice (frozen raindrops) bump into each other as they move around in the air. All of those collisions create an electric charge. After a while, the whole cloud fills up with electrical charges. The positive charges or protons form at the top of the cloud and the negative charges or electrons form at the bottom of the cloud. Since opposites attract, that causes a positive charge to build up on the ground beneath the cloud. The ground's electrical charge concentrates around anything that sticks up, such as mountains, people, or single trees. The charge coming up from these points eventually connects with a charge reaching down from the clouds and lightning strikes!
For complete details on lightning see the excellent article on How Stuff Works.
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- This type is rare. I have seen it once. When I was elementary school there was a thunderstorm and a ball passed through the window and exploded in a puff of smoke in the classroom. My dad also reported seeing it once when he was working on the railroad. A ball about the size of a basketball bounced down the track for some distance before it finally exploded. Yes, it DOES exist!