Clouds are primarily made out of water vapor and condensation nuclei.
How Clouds Form?
Water in clouds actually exists in two states, liquid and gas, and can also freeze into a solid, ice. When updrafts of air send water vapor high into the atmosphere, the vapor responds to the increasing cold by forming tiny droplets of water; this is “condensation”. These droplets actually need microscopic particles of dust or dirt to coalesce around, so each of the millions of drops of water making up a cloud contains tiny solid particles called condensation nuclei. 
Virga vs. Precipitation
Although these droplets are so small they can float on air currents, they do steadily sink. As they get closer to the ground and begin to warm, they evaporate. Once the droplets have returned to water vapor, they can begin the process all over. Droplets that almost reach the ground before evaporating are called “virga”, and form streaks beneath the cloud, while droplets that make it all the way to the ground are “precipitation”. Precipitation includes all kinds of rain, as well as sleet and snow. 
Evaporation Is Key to Cloud Formation
If clouds are made up of millions of droplets of water, and the droplets are created by water vapor turning to liquid and forming around dust, where does the water vapor come from in the first place? Evaporation is the process by which water is changed from a liquid to a gas called water vapor. The oceans account for eighty percent of all evaporated water, with inland water and vegetation accounting for the remaining twenty percent.  This evaporated water vapor forms clouds and eventually returns to earth as precipitation; the amount of precipitation is roughly equal to the evaporation, though there are geographical variations. 
It’s Relatively Simple
So, clouds are mainly water. Water evaporates into the atmosphere and forms droplets when it reaches colder air, and those droplets (along with a tiny amount of dust) form clouds. We perceive clouds as white because the tiny drops of water are all very reflective; clouds seem whitest on very sunny days.
 UCAR NCAR Science Education
Clouds and How They Form
 Texas A & M University Atmospheric Sciences
 WW 2010 University of Illinois
 USGS Water Science School
The Water Cycle: Evaporation