The best known type of lightning, called cloud-to-ground lightning, originates when ionized particles in a cloud form an electric field. It is unknown from where the negatively charged particles come from, probably from space in the form of solar wind. These ionized particles form a substance called plasma. (see here) This plasma in the cloud is negatively charged for normal lightning, and forms a large electric field. This electric field is strong enough to attract positively charged particles to cluster on the ground. As the storm moves across the sky, a group of particles follows a similar path along the ground. Soon, enough particles have accumulated and the negatively charged particles flow from the cloud downward. During this process, the flow of particles can split into many different paths, causing the phenomenon we know as fork lightning. We cannot see this part of the lightning's development, because the voltage is relatively low in this stage. In fact, the process that brings streams of negative particles down from the cloud taking a "long" time, relative to the flash, sometimes lasting up to a second. As the particles approach the ground, the attraction becomes powerful enough that the positive particles on the ground start to defy gravity, and soar upward. Then, the particles meet. Suddenly, the positively charged particles can neutralize their counterparts in the cloud easily by moving along the path. A huge movement of positive particles follows, resulting in a flow of energy 500 times larger than previously, heating the surrounding air to a remarkable temperature of 60,000 degrees. This flow, the original stroke of which lasts only a few milliseconds, is the flash we see. Additional flows of particles can occur in the next few hundred milliseconds, which add more flashes to the first.
When this extreme temperature hits the air, the velocity of the air molecules increases hundredfold, causing them to bump into other molecules, creating more heat. All this sudden movement is similar to an explosion, the sound of which creates thunder. Roars of thunder are longer than lightning flashes because the sound comes from each part of the lightning bolt. The sound from the ground area reaches you first, and then the sound from the upper part of the bolt. Additional lightning strokes cause prolonged thunder.
Although cloud-to-ground lightning is the type we know best, there are many different types. Cloud-to-ground lightning causes many visual effects which include the appearance of a ribbon, when lightning strikes move in a direction with each stroke, blending together into a "ribbon". Other effects also occur, such as forked lightning. However, this is all under the category of cloud-to-ground. Dry lightning, or lightning striking the ground without any precipitation, is also under this category.
A bolt of lightning with many forks. The forks are negatively charged paths that never connected with the group of positive particles on the ground. Some positive particles defied gravity and illuminated these forks, but most followed the main path, which is the "easiest" way to reach the cloud.
The most common type of lightning is not cloud-to-ground, but cloud-to-cloud lightning. Most lightning of this type is hidden by clouds, and causes the visual effect called sheet lightning. Hidden by clouds, only a flash can be seen from the lightning, which brightens all the clouds around it. Heat lightning is also a type of cloud-to-cloud lightning that has nothing to do with heat at all, but refers to the occurrence of lightning without accompanying thunder, which is a result of the lightning being too far away to hear.
There is also another type of lightning that is not a visual phenomena, but the nature of the lightning itself is different from normal. In normal lightning negatively-charged cloud bottoms are the source of the electric field. However, in the cloud itself a group of positive particles must exist to even out the charge. Usually this is at the top or the anvil of the cumulonimbus thunderstorm cloud. If this positive group of particles is extremely strong, which is necessary for the event, the negative particles accumulate on the ground and a lightning bolt goes all the way from the top of the cloud to the ground, which can be up to a ten mile span. Also, this type of lightning often travels up to thirty miles to strike an area that is ahead of the thunderstorm. Therefore, this type of lightning can occur when it is sunny, a trait that has earned it the name "bolt from the blue". The voltage on this positive lightning is ten times that of normal lightning and lasts for ten times longer, making this type of a hundred times more powerful overall.
A bolt of positive lightning, from the top of the cloud 50,000 feet high to the ground.
Also, many miles above the cloud tops, upper-atmospheric, or megalightning occurs, often mirroring lightning down below. This is probably due to the mirrored electrical charges that correspond to the lightning below. However, they come in different types. Sprites, which occur less than a second after their companion lightning strokes, are huge, and can occur up to 60 miles from the surface and appear a dim red color. Another sometimes related event is an elve, or a halo that sometimes occurs alone, and sometimes with a sprite. They are the same color as sprites, and are very faint. A third type that occurs in the upper-atmosphere are so-called blue jets that start about 9 miles up, follow a blue arc through the sky stretching to about 30 miles above the surface. The cause of this phenomenon is unknown, and these mysterious blue jets are elusive and difficult to record. Little is known about the nature of these transient luminous events (the name given to them that doesn't strictly relate them to lightning and puts them in a more unique category, because they lack many characteristics of normal lightning) and many people are researching these odd events.
A table comparing the different types of transient luminous events or upper-atmospheric lightning.
Lightning also can occur on other planets, and definite occurrences have been recorded on Jupiter and Saturn. Venus has also had lightning recorded on it by early probes but recently there has been a debate whether the probes actually detected lightning. Hopefully, this mystery will be resolved by future spacecraft.
Before the nature of lightning was known, many different civilizations thought to be struck by lightning was a punishment from the gods. If this was true, the gods obviously had a problem with Roy Sullivan, a man who, between 1942-1977 was struck by lightning seven times. This is a world record according to the Guinness Book of Records, which has stood since.
Lightning is an amazing event, to witness and to study. There is still a lot that we don't know about it that will hopefully be revealed in the future.
Sources: http://upload.wikimedia.org/wikipedia/commons/8/86/Darwin_storms.jpg (image), http://geology.about.com/od/sprites/a/sprites.htm, http://upload.wikimedia.org/wikipedia/commons/thumb/c/c4/Upperatmoslight1.jpg/400px-Upperatmoslight1.jpg (image)
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