Lightning and thunder are probably the 2 most incredible instantaneous physical atmospheric phenomena on the planet. 

I guess the question "what causes lighting? should have been asked first but that will be the next question"

Lightning heats the air to 50,000 degrees F. Heating air, causes it too expand. You can observe this at home, especially  in the Winter. When heating your house, the warmer air, because it expands, will become less dense and as a result, it rises. This will cause the air closer to the ceiling to be warmer and the 2nd floor to be warmer(or require less heating) because warmer air from the first floor is rising higher to the 2nd floor. 

If this effects air that has a temperature difference of just 10-20 degrees, imagine how much expansion takes place when the temperature difference is almost 50,000 degrees!

Enough for it to result in sort of an explosion. The sudden, heating of air from the ambient temperature to 50,000 degrees  results in a massive expansion of air in the narrow channel that defines the path of a lightning bolt. That rapid expansion of the air creates a sonic shock wave, similar to a sonic boom.......which is what we know as thunder. 

Thunder can actually cause property damage at close range from the pressure shock wave. It has been know to break windows. 

Thunder is a noise of course and travels at the speed of sound..............which is 767 miles/hour or 1,087 feet/second. There are 5,280 feet in a mile, so the speed of sound is basically 1 mile every 5 seconds.

Lightning travels at the speed of light..........186,000 miles/second(which is something like 8 times around the earth in 1 second).

As a result, you will see lighting immediately after it occurs..........but it takes the sound of thunder awhile longer to pass in the outward direction...........5 seconds to go a mile. This is why you can estimate how far the lighting was from you by counting how many seconds pass before hearing the thunder after you see it. 

If it takes 5 seconds, then it was a mile away. 1 second and it was just 1/5's  of a mile away...........less than 800 feet. If the lightning and thunder occur simultaneously and you are outside, then you might need to check your underwear (-:

https://www.youtube.com/watch?v=Ult4hwTpkhg

https://www.scientificamerican.com/article/what-causes-thunder/

https://en.wikipedia.org/wiki/Thunder

The Science of Thunder:

http://lightningsafety.com/nlsi_info/thunder2.html

During an 11-hour period on July 17-18, 2003, in a 15-mile (24-km) radius     centered in Merrillville, Indiana, there were 10,428 CG flashes. Since the     majority (between 50%-90%) of all lightning flashes are IC flashes that do     not strike the ground, taking a rate of 10 cloud flashes per CG flashes,     this storm had an estimated 104,280 flashes, which corresponds to 158 flashes     per min or 2.6 flashes per sec. 

For centuries there have been documented records from reliable individuals     reporting unusual behavior (anxiety, restlessness, and irritability) associated     with some pets and livestock prior to thunderstorm activity. This behavior     has been observed in animals for as much as an hour or more before the first     sound of thunder is heard in the distance. It is speculated that some animals     are reacting to hearing long-wave sound energy below the 20-Hz level from     an approaching thunderstorm.