Lightning forms when there is a charge separation. Thunderstorms build up a strong electromagnetic field, which causes molecules to be stripped of their electrons; the air becomes ionized. The positive ions (water/ice molecules that have lost their electrons) are moved to the tops of the clouds by the strong wind currents in the storm. Meanwhile, electrons gather on heavier ice particles that fall to the bottom of the clouds. This causes the charge separation: the tops of the clouds are positive, while the bottom is negative. Charge separation continues until the electrical potential becomes strong enough to allow current to flow. This rapid current flow is what we call lightning, and is very similar to the shock you get after rubbing your feet on a carpet and touching a metal object ... except that the current flow in lightning is massively greater. The description above can explain why a lightning discharge can take place between the top and bottom of the cloud. But lightning can also strike the ground ... what can cause this? Although the process is not completely understood, here's a simplified explanation: 
Storm clouds have a charge separation, with their bottoms being strongly negatively charged. This induces a positive charge in the ground below the clouds; the repulsive force of the negatively charged cloud pushes the electrons on the ground down and away from the surface, leaving the surface of the ground positively charged. The potential difference between the clouds and the ground can be as high as a billion volts. Voltages that are this high can easily pass through air despite the fact that air is a good insulator. (Static electricity 'shocks' can also jump through air; they too have a very high voltage ... but very small current). As the negative charge continues to build up in the bottom layers of cloud, the air below the cloud begins to become ionized, in long streamers called leaders. These negatively charged leaders grow longer, and soon begin to approach the ground, branching out as they go. Air in the leaders is ionized, (an ionized gas is called plasma), so this sets up an easy path for electrical current to follow. Leaders can approach ground level in as little as a tenth of a second, but very little current is travelling through them at this point , and they are all but invisible, being a faint purplish colour. On the ground, which is positively charged, leaders (called streamers) form from anything that sticks out of the ground, where the electric field is strongest ... trees and tall buildings, and even people. These positively charged leaders (paths of ionized air) begin to extend upwards from these objects. Meanwhile, the negatively charged leaders from the clouds continue to make their way downwards, and they may branch in all directions. These are called step leaders. 
When a positively charged leader extending upwards from the ground makes contact with a negatively charged leader dropping from the storm clouds, a continuous, highly conductive path is now available, and electrons begin to flow in a massive current along the leader. This electrical discharge rapidly superheats the air in the leader, causing it to radiate light. You see the light first because it travels very fast. The air around the flowing current also expands rapidly, producing a shock wave, which is heard as thunder. The rolling rumble of thunder is caused by the time delay as sound comes from different portions of the leader.As the current flows along the ionized path, it also travels down the other branches of the original leader, even though these branches did not themselves make contact. This is why a lightning strike has many branches.The entire process from the beginning of leader formation to the final surge of current takes about a third of a second. Many lightning strikes are actually a series of repeated electrical discharges down the same leader, one after the other, with a time interval of perhaps a twentieth of a second between strokes. Up to five strokes often occur in the same leader. 
A bolt of lightning can travel as fast as 160,000 km/h, and can be as hot as 28,000°C. A typical stroke of lightning carries an electric current of between 40,000 - 120,000 amps, and transfers enough electrical energy to power a 100 watt lightbulb for several months, perhaps trillions of watts. On average, there are over one hundred lightning strikes occurring somewhere on Earth every second. About 10% of all lightning is actually caused by a so-called positive stroke, where a discharge occurs from a positively charged portion of the cloud towards a negatively charged streamer from the ground. These positive strokes usually occur near the edge of a storm, and are much more powerful than regular lightning; they account for much of the damage to objects on the ground.
It has been discovered recently that lightning strokes also release bursts of high-energy gamma rays, possibly from their point of origin at the base of the storm cloud. The mechanism for this unusual energy release is currently under study. Protection: Lightning rods: Lightning rods on the top of high buildings concentrate the positive charge accumulating on the building at their tip, making it more likely that the cloud leader (and the resultant lightning strike) will make contact there. They are good conductors, so by connecting the lightning rod to a thick wire which enters the ground, the energy from the strike can be directed away from the building. 
Personal safety:Over a thousand people are struck by lightning each year in North America. If you are caught outside in a storm, find a building to enter as soon as possible. You should do this even if no lightning has been observed, since a lightning strike could happen at any time. Don't shelter under or near trees, as these are often the tallest things around and are often hit by lightning. If you can't find a building, don't lay down on the ground. You don't want the current from a nearby strike to pass through your body as it makes its way across the surface. Instead, put your feet together and crouch, but don't touch the ground with anything but your shoes. This makes it less likely that the passing current will flow into your body. If you are inside, don't use the phone; if lightning strikes the building it may flow through the phone lines and into your body. (Cordless or cell phones are OK). For a similar reason, stay away from plumbing fixtures, including toilets, bathtubs and faucets, unless you are sure the building uses PVC pipes rather than copper. Misconceptions About Lightning:
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