Clouds represent air that has risen and condensed water vapor into cloud droplets. Air rises when the upward directed pressure gradient force is greater than the force of gravity. An increase in the upward directed pressure gradient force can occur when the air is positively buoyant or when the air has been forced to rise.
Clouds tend to contain more moisture than the air surrounding the cloud. Water vapor is much lighter than diatomic Nitrogen or diatomic Oxygen. Since water vapor is less dense than dry air, moist air will have a tendency to float above drier air (all else being equal). Clouds float on drier air just as bubbles float on water.
The cloud droplets that make up a cloud are very tiny. Air movements have much more of an effect on moving cloud droplets as compared to gravity. Compare dropping a feather to that of dropping a baseball. The baseball will quickly fall to the ground while the feather will gradually flutter to the surface. Now compare a feather to a dust particle. The dust particle will take longer than the feather to reach the surface since even the slightest movement of air will keep the dust particle suspended.
Clouds do not have a tendency to sink since they are less dense than the air below or around them. Clouds that do sink will eventually evaporate since the sinking process produces a warming of the air. Warmer air can evaporate more moisture than colder air.