A cloud is a visible mass of water droplets or frozen ice crystals suspended in the Earth's atmosphere above the surface of the Earth or other planetary body

Low clouds 

Low clouds are made  up of  water droplets  

Cloud base from ground up to 8000 ft or 2500 m.
Low clouds consist of the famous cumulus and stratus clouds. They form either when the air has the correct humidity and pressure conditions or simply due heat building up during the day making air to rise, cool down and condense. Here you find the common 'cauliflower' cloud on a nice summer day but as well dark covered skies with rain or early morning fog.

Middle clouds
The middle cloud group consists of Altostratus and Altocumulus clouds. Middle clouds are made of ice crystals and water droplets. The base of a middle cloud above the surface can be anywhere from 2000-8000m in the tropics to 2000-4000m in the polar regions.

High clouds

The high cloud group consists of Cirrus, Cirrostratus, and Cirrocumulus clouds. High clouds are made of ice crystals due to the cold air in the upper sky. The base of a high cloud above the surface can be anywhere from 6000-18000m in the tropics to 3000-8000m in the polar regions.