it is a natural electrical discharge of very short duration and high voltage between a cloud and the ground or within a cloud, accompanied by a bright flash and also thunder. the whole cloud fills up with electrical charges. lighter, positively charged particles form at the top of the cloud. heavier, negatively charged particles sink to the bottom of the cloud. when the positive and negative charges grow large enough, a giant spark - lightning - occurs between the two charges within the cloud. This is like a static electricity sparks you see, but much bigger.