Thunder is the sound made by lightning.
Depending on the nature of the lightning and distance of the listener, thunder can range from a sharp, loud crack to a long, low rumble.
The deep rumbling and sharp cracks of thunder are produced as the air around the lightning bolt is super heated up to 60,000 degrees Fahrenheit (33,315 C), which then rapidly expands.
In turn, this expansion of air creates a sonic shock wave which produces the sound of thunder.
The closer the lightning is, the louder the clap of thunder will be.
The distance of the lightning can be calculated by the listener based on the time interval from when the lightning is seen to when the sound is heard.