Cumulonimbus are massive, convective, rain-producing clouds with substantial vertical extent and storm potential. They form in unstable, moist conditions and were formalised in cloud atlases; they appear frequently in art and storm literature.
Cumulus are low, puffed, convective clouds formed by surface heating and uplift. They have flat bases and variable tops. The name was coined in the early 19th century and is widely familiar in art, and literature.