Cumulus

Cumulus clouds are the clouds that look like cotton balls or the puffy clouds. 

Cumulus means "Heap" or "pile" in latin.

They can occur with severe weather.

They are part of a large category called Cumuliform clouds.

The way that Cumulus clouds form when warm air rises and reaches the point where it meets cool air.

Why they form is that they are a precursors,before/warning, of clouds like the cumulonimbus clouds.