A lightning belongs to the group of meteors other than clouds, the so-called electrometeors. Lightning is a luminous manifestation accompanying a sudden electrical discharge which takes place from or inside a cloud or, less often, from high structures on the ground or from mountains. 

Three main types of lightning can be distinguished:

1) Ground discharges (thunderbolts) - CG: this type of lightning occurs between cloud and ground, it follows a tortuous course and is usually branded downward from a distinct main channel.

2) Cloud discharges (sheet lightning) - CC: this type of lightning takes place within the thundercloud. It give a diffuse illumination without a distinct channel being usually seen. This type of lightning includes the so-called heat lightning, consisting of diffuse light flashed from distant thunderstorms, seen at horizon.

3) Air discharges (streak lightning) - CA: this type of lightning occurs in form of sinuous discharges, often ramified but with a distinct main channel, passing from thundercloud to the air and not striking the ground. It frequently includes a long quasi-horizontal part.

  

The picture was taken on the back of a large thunderstorm complex. The lightning is composed of: on the right a forked CG lightning and on the left a long horizontal CA lightning. This lightning does not only illuminates the clouds above but also the falling rain below.