Lightning is a massive electrostatic discharge
 between electrically charged regions within clouds
in simple words 
Lightening  happens between  a cloud and the Earth's surface.

There are 3 primary types of lightening 

1.Intra cloud (IC)
 lightening forms with  in cloud itself

2.Cloud to another  cloud (CC)
This is between 2 clouds 

3. Cloud and Ground (CG)
between cloud and the ground

                         Lightning occurs approximately 40–50 times in a second worldwide
  1.4 billion light  flashes per year

Power of  1 lightening  is nearly  3 million volts