There are three main types of lightning: cloud to ground cloud to cloud intercloud In a clear calm atmosphere, electric charge is fairly uniform in the atmosphere. During a thunderstorm, due to the physical processes going on within and around a cloud, areas of the cloud build certain types of charge. The cloud to ground type of lightning, the most interesting and frightening, happens when a negative charge builds up in low levels of a cloud. The surface of the Earth is positively charged. Since these opposite charges attract, when the negative charge builds to a large enough point in the cloud to overcome air resistance, it rushes towards Earth's surface (stepped leader) and a positive charge rushes up from the ground to meet it. When these two charges meet, positive charge is transferred to the cloud, and a flash of lightning, visible to the human eye, is born.

Great answers. I just wanted to add that there is another type of lightning, Cloud To Air

Here are some dot jots about how lightning is caused, Lightning originates from the ground. Raindrops are carried upward until some raindrops convert to ice. A cloud-to-ground lightning flash originates in this mixed water and ice region. The electric charge then made travels down until it hits the ground. It produces a channel when it along where the charge is desposited.

Electrical charges (pluses and minuses associated with electrons, protons, and ions) are present in roughly equal amounts, keeping most things neutral. In thunderstorm weather the charges get separated; the clouds acquire a different amount of charge than the ground does. This represents an enormous amount of static electricity. A large voltage exists between the cloud and ground. Lightning is essentially a giant spark similar to the little static shocks we all get in the winter. Lightning is a plasma -- a hot state of matter where the molecules are ionized. They get stripped of electrons by the high electric field (voltage gradient). The plasma sustains extremely high temperatures and conductivities until neutrality is once again reached, after which the air molecules return to the neutral gas state. Meanwhile a lot of light gets radiated from the plasma -- that's what we see as "lightning". Also the air adjacent to the plasma gets shocked and emits a large sound wave in all directions -- that's the "thunder." Lightning is actually more complex than a "giant spark." It carves out a path through many layers of atmosphere over a very brief time, all the while branching, changing direction, etc. Some lightning forms at the ground and works upward to the cloud, rather than vice-versa.

i still don't det why the positive and negative charges separate.

Throw a football in the air and it creates lightening.