From SKYbrary Wiki
Cloud is the visible collection of very small particles of water and/or ice in the air.
As a parcel of air rises, it cools. Cooler air can support less water vapour and so, as the air cools, the relative humidity of the air increases. The temperature at which the air humidity reaches 100% is known as the Dew Point. At this point the air is said to be saturated. As the temperature continues to fall, the excess water vapour that the now saturated air contains condenses to form water droplets or ice particles. Clouds therefore form when the relative humidity of a parcel of air reaches 100%.
What form the clouds take will depend on the stability of the atmosphere - for further information see Cloud Types.