Short answers, pulled from the story.
A cloud is an aerosol, a visible mass of miniature liquid droplets, ice crystals, or other particles suspended in the atmosphere of a planetary body. On Earth the droplets and crystals are made primarily of water. Clouds are seen in the homosphere, which includes the troposphere, stratosphere, and mesosphere.
Luke Howard, a methodical observer in England, formally classified the tropospheric cloud types during 1802, using Latin names that were published in 1803. Jean-Baptiste Lamarck devised a competing French scheme the same year, but it failed to catch on. An elaboration of Howard's system was formally adopted by the International Meteorological Conference in 1891.
The ten tropospheric genera derive from crossing five physical forms with altitude levels. The forms are stratiform sheets, cumuliform heaps, stratocumuliform bands or ripples, cumulonimbiform towers, and cirriform wisps. Mid-level types take the prefix alto- and high-level types take cirro-, while low-level types carry no altitude prefix.
Clouds form when air becomes saturated, either by cooling to its dew point or by gaining enough moisture to raise the dew point to the air temperature. Most formation happens through adiabatic cooling driven by convective, frontal or cyclonic, or orographic lift. Condensation usually occurs on cloud condensation nuclei such as salt or dust particles.
Fog and mist both occur when a very low stratus cloud sinks to surface level and loses its Latin name. It is called fog when surface visibility is less than 1 km, and mist when visibility is 1 km or higher. Both are considered surface-based cloud layers with no Latin nomenclature.
Clouds both cool and warm the planet, which makes them the main uncertainty in climate sensitivity. White cloud tops reflect shortwave sunlight and raise Earth's albedo, cooling the surface, while cloud water absorbs infrared radiation and re-emits it downward, warming the surface. NASA measurements indicate the cooling effect of low and mid-level clouds outweighs the warming effect of high clouds.
Yes, cloud cover has been seen on most other planets in the Solar System, though often made of substances other than water. Venus has stratiform sulfur dioxide clouds, Jupiter and Saturn layer ammonia, ammonium hydrosulfide, and water clouds, and Uranus and Neptune have methane clouds. In 2013, high-altitude optically thick clouds were announced on the exoplanet Kepler-7b.