Cumulus clouds are detached clouds that often resemble cauliflower.  They are "puffy" in appearance, generally dense and with sharp outlines.  They develop vertically in the form of rising mounds, domes, or towers.

The sunlit parts of these clouds are a brilliant white, but the bottoms are darker and the bases have a distinctly "flat, horizontal" appearance.

Cumulus clouds can simultaneously occur in various stages of vertical development. 

Cumulus is composed mainly of water droplets; ice crystals may form in the upper parts of the clouds, which are below 0 degrees Celsius.  Cumulus of great vertical extent can produce showery precipitation.

Cumulus clouds are considered to be convective.  They develop in convection currents which occur when the lapse rate in lower layers is sufficiently steep.