Before Luke Howard developed his system for naming clouds, clouds were spoken of as ‘essences’ floating across the sky, and believed to be impossible to categorise and name. He recognised the need for a universal language for this global phenomenon and chose Latin, the language used by Linnaeus for his classification of plants and animals.

Luke Howard identified three main categories:
Cumulus (Latin for heap)
Convex or conical heaps, increasing upward from a horizontal base – cotton wool clouds

Stratus (Latin for layer)
A widely extended horizontal sheet, increasing from below

Cirrus (Latin for curl of hair)
Parallel, flexuous fibres extensible by increase in any or all directions
and an additional fourth category:

Nimbus (Latin for rain)
A rain cloud — a cloud or systems of clouds from which rain is falling

Origin of the word – Cloud. 
In the ninth century it was a noun meaning a pile of rocks or a hill (cloud is related to clod or clot). Around 1300, clouds became (as they still are) heaps in the sky. Then in the 16th century, cloud was converted into a verb, meaning to “darken” or “obscure”.
Guardian Review  26/3/16, Mind your pease and trews by Rebecca Gowers

Cloud Appreciation Society

Luke Howard’s cloud classifications were extended later in the century, to include the types of clouds that are still in use throughout the world today.

The World Meteorological Organisation recognizes ten cloud genera, or basic classifications, which describe where in the sky they form and their approximate appearance:

          High clouds (CH): Cirrus, Cirrocumulus, Cirrostratus
          Middle clouds (CM): Altocumulus, Altostratus, Nimbostratus
          Low clouds (CL): Stratocumulus, Stratus, Cumulus
          Cumulonimbus

These can then be further broken into 14 different species, nine different varieties, six different supplementary features and three different accessory clouds leading to approximately 100 combinations.

The official WMO International Cloud Atlas is now available online -
"We hope this website inspires you to become even more enthusiastic about observing clouds and all atmospheric phenomena".

For more information and to see wonderful photographs of clouds from across the globe – go to Cloud Appreciation Society

The sky too belongs to the Landscape: – the ocean of air in which we live and move, with its continents and islands of cloud, its tides and currents of constant and variable winds, is a component part of the great globe; and those regions in which the bolt of heaven is forged, and the fructifying rain condensed, – where the cold hail concretes in the summer cloud, – and from whence large masses of stone and metal have descended at times upon the earth, can never be to the zealous Naturalist a subject of tame and unfeeling contemplation.
Luke Howard, Seven Lectures on Meterology, 1837