Cumulus clouds are clouds which have flat bases and are often described as "puffy", "cotton-like" or "fluffy" in appearance. Their name derives from the Latin 'cumulo-', meaning heap or pile.

Cumulus clouds are often precursors of other types of clouds, such as cumulonimbus, when influenced by weather factors such as instability, moisture, and temperature gradient. Normally, cumulus clouds produce little or no precipitation, but they can grow into the precipitation-bearing congestus or cumulonimbus clouds.

Cumulus clouds can be formed from water vapor, supercooled water droplets, or ice crystals, depending upon the ambient temperature. They come in many distinct subforms, and generally cool the earth by reflecting the incoming solar radiation.

Stratus clouds mean rain if it is warm and snow if it is cold. They look like a huge gray blanket that hangs low in the sky. Sometimes stratus clouds are on the ground or very near the ground, and then we call them fog.

Cirrus Clouds thin and swirled. The most common form of high-level clouds are thin and often wispy cirrus clouds. Typically found at heights greater than 20,000 feet (6,000 meters), cirrus clouds are composed of ice crystals that originate from the freezing of supercooled water droplets.

Nimbus clouds are clouds that produce precipitation that reach the ground in the form of rain, snow or hail. They are usually grey and low in the sky.

More Info: en.wikipedia.org