Clouds are made of drops of water or ice. When the wind picks up water from the ocean or from the land on Earth, it can carry the water up into the sky. If there's a lot of water drops together, most of the sunlight that hits the cloud reflects off the water or ice and comes to your eye, where it looks white. In a thick cloud, the water drops scatter the light in all directions instead of reflecting it, and the cloud can look gray or black.
Clouds can happen at any height above the ground. Sometimes clouds are actually touching the ground - we call that fog. If you go up to the top of a mountain or in an airplane, you will be above most of the clouds. After it rains, the clouds disappear, because the water they were made of has all come back down to Earth again.
On Earth, there are four main types of clouds:
On other planets, there can be clouds made of other things than water. Venus is covered by thick clouds, but the clouds there are made of drops of sulphuric acid, chlorine, and fluorine, and would be poisonous to you.