(a) Electrostatics

Lightning is caused by the build up of electrostatic charge in clouds. One region within the cloud builds up a positive charge and the other a negative charge. If the build up (separation) of charge becomes great enough, the negative charges may leap to the positive side of another cloud, this is called sheet lightning. The accumulated charges may ionize the air between the charged cloud and the ground. These ionized air will then provide a conducting path for charges to be discharged to the nearest or sharpest object on ground.

Lightning may cause great destruction. How do we take precaution against destructive effects of lightning? Lightning conductors are fitted on top of tall buildings to provide a discharge path for excessive charges in the air to flow from top of the building into earth.

http://www.howstuffworks.com/lightning.htm