http://www.livescience.com/3803-science-lightning.html

Most of the electrical  energy in a thunderstorm is dissipated within the clouds, as lightning hops  between the positively and negatively charged areas. Lightning becomes dangerous,  though, when it reaches for the Earth.

How  lightning strikes

When the negative charge in the cloud becomes great enough, it seeks an easy path to the positively charged ground below. The current looks for a good conductor of electricity, or a tall structure anchored to the ground (such as a tree or a tall building). The negative charge sends out a feeler, called a stepped  leader, which is a series of invisible steps of negative charges.

As the stepped leader  nears the ground, a positive streamer reaches up for it. Only then, once  this channel is made, does the visible lightning happen. A return  stroke runs from the ground to the clouds in a spectacular flash. 

Though the bolt appears continuous, it is actually a series of short bursts. Most lightning strikes occur in less than a half second and the bolt is usually less than 2 inches in diameter.

http://www.livescience.com/3803-science-lightning.html

Dark

Some lightning can’t be seen. Gamma rays burst from thunderheads without giving off heat or light, but their radiation is 100 times more energetic than a medical x-ray. Scientists were perplexed by these invisible flashes until researchers recently found that they, too, diffuse imbalanced charges in clouds.

Alien

Extraterrestrial lightning is always tricky to observe. But during Saturn’s 2009 equinox, conditions dampened the rings’ glow enough for the Cassini spacecraft to capture flashes on the gaseous planet. Lightning has also been detected on Jupiter, and radio data suggests it exists on Uranus and Neptune.

Ball

Named for the bizarre orbs that have been spotted during storms, ball lightning was finally caught on camera by scientists in 2012. “We don’t know what it is,” says lightning researcher Don MacGorman. Spectral analysis suggests the balls form when lightning strikes and vaporizes elements in the soil.

Volcanic

So-called dirty thunderstorms are thought to occur when dust particles from a volcanic ash plume collide with ice crystals in the atmosphere. This event is hard to study, so researchers in Germany created a plume in the lab using real volcanic ash. The smaller the particles, the more prolific the lightning.