When a lightning bolt strikes heats the air to such extreme that it immediately expands sending vibrations or shock waves, after this we hear the explosion of sound.
Thunder is the sound formed by explosive expansion of air that is heated by a lightning stroke to temperatures as high as 30,000°C. Thunder is still not completely understood by scientists today. But they do know that it is caused when the heat released by the lightning bolt.
Lightning is the release produced when voltage differences between the ground and atmospheric are big enough to overcome the insulating result of the air. Lightning strokes can take place within a cloud, among clouds or between clouds and the ground.