Clouds are made up of tiny water droplets and ice crystals that are so small they can float in the air.  If the droplets become large enough, you will be able to see them as cloud or fog. If they become even larger, they can fall as rain (or snow).

The water in a glass, like the one shown below, might not look like it is moving at all, but the molecules that make up the water are always moving. When the water is warm, the molecules move faster. When the water is cool, the molecules move more slowly. Some of the molecules are moving fast enough that they escape into the air. When water molecules move from the glass into the air, the water is evaporating. If we heat the water, more water molecules evaporate becoming water vapor.  If the air cools, then water vapor molecules slow down and some can not remain a vapor. They cluster in the air to form tiny liquid droplets. This is called condensation. In clouds, the liquid droplets formed by condensation are small and light enough that they stay in the air. If they grow large enough, they will fall to the ground as precipitation.