Clouds are classified according to their height above and appearance (texture) from the ground. The following cloud roots and translations summarize the components of this classification system: 1) Cirro-: curl of hair, high; 2) Alto-: mid; 3) Strato-: layer; 4) Nimbo-: rain, precipitation; and 5) Cumulo-: heap.

Most clouds can be divided into groups (high/middle/low) based on the height of the cloud's base above the Earth's surface. Other clouds are grouped not by their height, but by their unique characteristics, such as those that form over mountains (lenticular clouds) or beneath existing clouds (mammatus clouds).

It became the basis of a modern international system that divides clouds into five physical forms which can be further divided or classified into altitude levels to derive ten basic genera. The main representative cloud types for each of these forms are stratiform, cumuliform, stratocumuliform, cumulonimbiform, and cirriform.