Clouds are made of water. When water is vaporized (from boiling it or from evaporation) it goes into the sky as a gas. If there is enough water in the sky, other things in the sky (like dust), will cause the water to condense, or stick together. This accumulation of droplets/crystals is lighter than air and so it floats, making the cloud, and once it gets too heavy, it falls from the sky as rain or snow.