Of all the clouds that appear in the sky, cumulonimbus clouds are the ones that garner the most attention from people on the ground, and are often the ones that are most readily named. There's a good reason for this, too, for depending on your perspective cumulonimbus clouds typically bring with them bad news.
Cumulonimbus clouds, like all varieties of clouds, derive their name from two combined Latin words. "Cumulo", coming from the cumulus family of clouds, refers to their bulky, thick, piled appearance that typically stretches high into the sky, making them vertical clouds with very low bases. "Nimbus" refers to their capacity for bringing with them precipitation, typically rain - and, more often than not, storms. Cumulonimbus clouds typically have darker bottoms than the vast majority of other clouds, a consequence of their capacity for massive rain showers that usually end in thunder and lightning.
As mentioned, cumulonimbus clouds appear as huge, heaping mounds of black and white in the sky, shaped like anvils or, in some cases, mushrooms reminiscent of atom bomb explosions. The precipitation that drags them down through the troposphere turns the bottom of the cloud near black, signifying the congregation of water droplets that eventually fall as rain. The peak of the cloud, which can stretch thousands of feet higher than the base and may not be visible from the ground at all (cumulonimbus clouds often travel in packs), is white when viewed from a distance or from a plane, a consequence of ice crystals concentrated at the top.
Why cumulonimbus clouds are so often paired with lightning as well as rain isn't yet concretely known since the process by which lightning is generated remains a matter of contention. Given that lightning is often hypothesized as the interaction between water droplets and ice crystals inside the cloud this isn't surprising, however, since the sheer vertical size of cumulonimbus clouds allows them to stretch from warm air to cold. The existence of so much water and ice in one cloud may be enough to do the job.
Cumulonimbus clouds form in warm weather regions and during the summer when and where the Earth is at its hottest, the difference in temperatures from passing weather fronts converging to create these huge storm clouds. In particularly hot weather these clouds can form with surprising rapidity, stretching the life of storms that might only last for half an hour to hours on end. In extreme situations cumulonimbus clouds can result in supercell storms or even tornadoes, the latter of which is unsurprisingly found in dry, warm, flat areas like the American Southwest (the infamous 'Tornado Alley').
Because of the nature of their storm activity, cumulonimbus clouds are always associated with rain and storms. Cumulonimbus clouds that do not create precipitation are simply cumulus clouds, and typically appear much lighter gray at the base.
Wikipedia: Cumulonimbus Clouds