A big, bright bolt of lightning streaks across the sky. A few seconds later, you hear the rumble and crash of thunder as it shakes the windows in your house. The sky is dark and the rain is pouring down. Those gloomy clouds keeping you from playing outside and having fun are called cumulonimbus clouds.

Cumulonimbus clouds are large, tall clouds that are dark on the bottom and usually produce rain and thunderstorms. In fact, they are sometimes called thunderstorm clouds, but they can also bring different kinds of weather, including hail and snow showers, which might even get you out of school for a snow day!
http://www.metoffice.gov.uk/learning/clouds/low-level-clouds/cumulonimbus
https://en.m.wikipedia.org/wiki/Cumulonimbus_cloud

Cumulonimbus, from the Latin cumulus ("heap") and nimbus ("rainstorm", "storm cloud"), is a dense towering vertical cloud associated with thunderstorms and atmospheric instability, forming from water vapor carried by powerful upward air currents. If observed during a storm, these clouds may be referred to as thunderheads. Cumulonimbus can form alone, in clusters, or along cold front squall lines. These clouds are capable of producing lightning and other dangerous severe weather, such as tornadoes. Cumulonimbus progress from overdeveloped cumulus congestus clouds and may further develop as part of a supercell.