Clouds form when moist air cools to its dew point. The water vapour then condenses into droplets or ice crystals.

You sort clouds by the height of their base: low, middle and high. In mid-latitudes low clouds are below about 6,500 ft and high clouds above about 20,000 ft. Some clouds grow through every band.

The names tell you what they look like: stratus is a layer, cumulus a heap and cirrus a wisp. Alto means middle and nimbo means precipitation.