The dictionary defines cloud as "a visible mass of condensed watery vapour floating in the atmosphere, typically high above the general level of the ground."
Clouds are made up of a mixture of water droplets, ice crystals and water vapor that are floating up in the atmosphere. When warm air rises, it cools. As the warm air cools, it causes condensation. This condensation floats around in the atmosphere clumping together and creating clouds.
Different types of clouds are formed due to factors such as height and density/amount of water droplets, ice crystals and water vapour in atmosphere.