We all know that in the earth gravity is there, but why clouds doesn't fall down?

Answer: Clouds are evaporated sea water. You should be asking "Why does the gravity on Earth pull us down and not evaporated water.

Because of their mass

Explanation:

Clouds are made of water right? so you are asking why they don't fall down if air is lighter than water.

The two biggest reasons the clouds stay in the sky are

1. Small Drops

2. Wind

Small drops of water fall more slowly than big drops, as the drops of the clouds fall thought the air, the air pushes back to them, because Small drops has less mass than large drops, clouds can only contain small drops of water.