Clouds form when the H₂O vapor that you cannot see in the air condenses to form TINY TINY droplets. These form in regions where the air is moving UPWARDS, because as the height above the ground increases, the temperature gets colder, and that makes the water that is dissolved in the air to CONDENSE from the gas to the liquid... Now, the boundary between the two regions, or the CONDENSATION FRONT can be very sharp, only a few feet in distance; this makes clouds appear fluffy but also sharp.

They're expanding into the cooler air around them, which gives them a boiling look.

There are many types of clouds, but the fluffy ones you're asking about are called cumulus. Clouds are made up of small droplets of water (if the clouds are near the equator) or ice crystals (if they are at higher latitudes) when warm, moisture-laiden air rises into the atmosphere (low pressure zone at Earth's surface). As that warm, moist air rises, the water condenses because the temperature of the atmosphere decreases moving away from the surface. So, as the warm, moist air is rising, and water droplets or ice crystals are condensing, the mass of air expands in volume, giving a "fluffy" appearance.

The condensed water looks white because it reflects all the light from the sun. Cumulus clouds generally form "low" in the atmosphere, but if they grow large enough, they may form an "anvil" shape because the top of the cloud is moving into a different layer of the atmosphere with faster-moving winds. If a cumulus cloud grows large enough in the right conditions, it can become a rain cloud, which gets a different name, cumulonimbus.