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.
The cumulonimbus base may extend several miles across and occupy low to middle altitudes - formed at altitude from ~200 to 4,000 m (700 to 10,000 ft).
Peaks typically reach to as much as 40,000 ft (12,000 m), with extreme instances as high as 70,000 ft (21,000 m) or more.
Well-developed cumulonimbus clouds are characterized by a flat, anvil-like top (anvil dome), caused by wind shear or inversion near the tropopause. The shelf of the anvil may precede the main cloud's vertical component for many miles, and be accompanied by lightning.
Reference: Wikipedia