Cirrus: Loosely-spaced fluffy clouds indicating fair weather.

Cumulus Clouds, by oatsy40, licensed under CC BY 2.0

Cumulonimbus: Massive multi-level formation indicating thunderstorms.

Clumped, by Nicholas_T, licensed under CC BY 2.0

Onthedoggiebus: Loosely-spaced chaotic formation indicating trip to the dog park.