Weather is a big part of flying, and clouds are a big part of weather. All pilots need to understand how clouds form and how they behave. If you are in the process of learning to fly you need to know the same information well enough to pass the written Private Pilot License exam.
For the purpose of the Canadian PPL exam there are four types of cloud: high, middle, low and clouds of vertical development. The first three categories depend on the height of their bases. The fourth type of cloud can extend through the entire range of cloud heights, with their bases beginning in the low or middle cloud range.
High clouds have bases beginning above 20,000 feet above sea level. Their names contain either the word "cirro" or "cirrus". Cirrus clouds have a streaky, fibrous, layered appearance. Cirrostratus clouds are long thin layers - "stratus" means, in this case, long and flat looking. Cirrocumulus clouds are smaller, puffy clouds. They look a little flatter than other puffy clouds ("cumulus" is the term for puffy clouds of one type or another).
The bases of middle clouds form between 6,500' and 20,000'. These cloud names are pre-fixed with "alto". Altostratus clouds are a uniform layer stretching large distances. Again, "stratus" indicates a layered quality. Altocumulus clouds are puffy clouds that cover less area then altostratus clouds. "Alto" means mid level and "cumulus" means puffy. Altocumulus castellanus is the name used for altocumulus clouds that grow vertically to significant heights. Often the tops of ACC clouds flatten out because of high altitude winds.
Low clouds form their base below 6,500'. Stratus clouds are (as the name implies) flat layers of low cloud. Nimbostratus are flat layers of low cloud with a high moisture content. They produce lots of rain, sleet, snow or freezing rain. Fractostratus or Stratus Fractus clouds is a stratus cloud (a uniform layer) broken with gaps (hence the addition of the term "fractus"). Cumulus clouds are puffy low clouds. Note that mid and high level clouds get the appropriate modifier ("cirro" or "alto") but low clouds do not.
Clouds of vertical development are of particular concern to pilots. They have enough energy and can change so quickly that they pose a serious threat to aviators. They can begin forming in the low cloud range (below 6500') or in the middle cloud range (6500'-20,000'), and can top out in the stratosphere. They begin with cumulus clouds that develop into towering cumulus clouds or cumulonimbus clouds ("nimbus" indicates precipitation of some sort). The updrafts and downdrafts in towering cumulus clouds can make flight impossible. Even large airliners avoid these clouds.
Clouds form when the water vapor in air condenses. This occurs because air cools as altitude increases. Low pressure zones are often cloudy, as water vapor flows into low pressure areas as the atmosphere tries to balance itself (high pressure zones flow toward low pressure zones). This is an ongoing process because high and low pressure zones are constantly forming and breaking down.
Forecasters and weather reporting stations determine coverage by dividing the sky into eight parts (oktas). A clear sky has no clouds below 10,000. An SKC sky is clear completely. A sky described as "Few" mean coverage is greater than zero oktas but does not exceed 2 oktas (2/8s). Scattered means a sky with coverage of 3 to 4 oktas (3/8s - 4/8s). A broken sky is covered from 5 oktas to just less than 8 oktas (5/8s to just under 8/8s). An overcast sky is 8 oktas.
Mist and fog are just clouds that form at ground level ("F"ormed "O"n "G"round = fog). The visibility in mist is 5/8 of a nautical mile or more, while visibility in fog is less than 5/8s of a nautical mile. Visibility is the only difference.
It is important to understand these concepts. In the first place, understanding them will allow a pilot to predict what sort of weather he is likely to encounter during a flight. Second, the pilot needs to be able to recognize them, especially TCUs, in order to avoid danger. Last, the student pilot needs to know this information in order to pass the written test.