A cloud is a part of the air where there are so many tiny water droplets that they make it impossible to see through. That is why people say that fog is cloud at ground level.

If you fly in an airplane, you will sometimes fly through clouds and notice that the visibility suddenly gets very poor.

Clouds are formed when moist air cools down. Clouds like cumulus clouds are formed on hot days when invisible bubbles of air rise from a hot land. They give individual clouds, and sometimes showers. If they grow to be very big they give thunderstorms and even tornadoes.

Clouds can sometimes been formed when cold air is pushed off the ground in a great layer. This happens in a place that weather forecasters call a low pressure or depression.

It's all very complicated, especially as you can't see the air moving before it makes cloud.