What is a lightning strike ?

A lightning strike is an electrical discharge generated by an electrified cloud (usually cumulonimbus) between itself and the ground (approximately 10% of cases only), inside the cloud (intra-cloud flash) or between adjacent clouds (inter-cloud flash).

 
Where does the roll of thunder come from ?

The noise we all associate with a thunderstorm is caused by a rise in the electrodynamic pressure estimated at 2 or 3 atmospheres. The excess pressure drops when the flash is over, producing an acoustic shock wave. The duration and the acoustic intensity of the roll will depend on the form, length, intensity and brevity of the lightning strike.

How many times is the Earth struck by lightning over a 12-month period ?
On average, there are several hundred flashes every second.