Clouds are water which has been evaporated and exist because the dew point at altitude is lower, which means that water turns into vapor (which is why your breath clouds up when it gets cold). The air temperature determines how much water air can hold, and any amount of water beyond that showers up as clouds.
Vossenoren t1_j6lozge wrote
Reply to ELI5 What are clouds made of? by MrBoneStealer
Clouds are water which has been evaporated and exist because the dew point at altitude is lower, which means that water turns into vapor (which is why your breath clouds up when it gets cold). The air temperature determines how much water air can hold, and any amount of water beyond that showers up as clouds.
It's water. Water, basically.