The cloud base is the distance from the ground to the bottom of a cloud. Clouds often have fairly flat bottoms (especially large cloud masses and clouds produced by convective lifting). A cloud typically forms when rising air becomes saturated enough to start condensing out water vapor. Since the parcel of air that produces the cloud has a similar amount of moisture throughout, the moisture will begin condenses out at a similar elevation above the ground.

A cloud base is determined by the initial amount of moisture in the rising parcel of air. If a parcel of air starts off fairly dry (i.e. RH of 20%), the parcel will have to rise much higher than if the parcel of air starts off fairly moist (i.e. RH of 80%). See diagram below.

All types of clouds have cloud bases and there can be multiple cloud bases. Often the most referenced cloud base to consider is the first cloud base encountered above the surface. Typically this cloud base is determined by parcels being lifted from the surface (although there are many exceptions). Thus, the moisture characteristics of a parcel lifted from the surface will determine how high above the surface the cloud develops.