A cloud can contain an enormous amount of water.
So why does it not fall out of the sky all at once?
Because that water is divided among billions of extremely tiny droplets or ice crystals.
Cloud droplets are tiny
A typical cloud droplet is so small that its fall through air is extremely slow.
Air resistance has a large effect on something with so little mass.
The droplet may technically be falling relative to the surrounding air, but very slowly.
Air itself is moving
The atmosphere is not stationary.
Rising currents can carry cloud droplets upward.
Turbulence moves them around.
As long as upward air motion balances or exceeds their slow downward settling, the droplets remain suspended.
How does a cloud form?
Moist air rises or cools.
When it cools enough, water vapor condenses onto tiny airborne particles.
The result is microscopic liquid droplets or ice crystals.
This is closely related to why fog forms. Fog is essentially a cloud forming close to the ground.
When does the water finally fall?
Cloud droplets collide and combine.
Ice crystals can grow.
Eventually some particles become large enough that gravity wins decisively.
They fall as rain, snow, sleet, or another form of precipitation.
Why can clouds look so heavy?
The total water mass can be enormous even while each individual droplet is tiny.
Think of a cloud as a huge region containing countless small particles spread through a large volume of air.
The total is impressive.
The individual pieces remain easy for air currents to support.
Are clouds water vapor?
Not the visible part.
Water vapor is invisible.
The cloud you see consists of liquid droplets, ice crystals, or both.
That distinction is also why fog is made of tiny droplets rather than visible water vapor.
So a cloud may contain tons of water.
It stays up because that water has been divided into billions of tiny pieces moving with the air.
And that’s the short explanation.
Related explanations: Why does fog form?, Why does rain smell so good?, and Why is snow white if ice is clear?