Have you ever noticed that sometimes thunder is a sudden, loud crack while at other times it gently rolls across the sky? These two different sounds are both sounds made by a lightning strike. Thunder is just the shock wave made when lightning suddenly heats the air to 40,000 to 50,000 degrees Fahrenheit.
If the lightning strikes nearby, you hear the boom of the sudden expansion of the air. If the lightning is some distance away, you hear the rolling of that shock wave across the sky. In some cases, you may hear both. Sometimes, some or all of the sound can be absorbed, blocked, or reflected by rainfall.
When a thunderstorm produces really intense lightning, the booms and rumbles can become continuous and can meld into a too-loud symphony of storm.