Cumulonimbus clouds are associated with heavy rain, snow, hail, lightning and even tornadoes. The anvil usually points in the direction the storm is moving. Mammatus clouds are low hanging bulges that droop from cumulonimbus clouds. Mammatus clouds are usually associated with severe weather.
If you're talking about cumulonimbus clouds, they are usually bring about thunderstorms. Most of the time, they are associated with weather such as heavy rain, snow, hail, lightning, and depending on temperature conditions and wind speed, they can even bring on tornadoes.
