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What is a cloud?
A cloud is simply a visible mass of tiny water drops and/or ice crystals. Clouds form when the relative humidity reaches or exceeds 100%. The relative humidity is the ratio of the amount of water vapor in the air compared with the amount of water vapor that is possible to exist in the air. At sea level and a temperature of 25°C (77°F), a maximum of 23 grams of water vapor can exist per cubic meter. If the air does indeed contain 23 grams of water vapor per cubic meter, then the relative humidity is 100% and the air is said to be "saturated." The maximum amount of water vapor possible in 0°C (32°F) air (at sea level) is only 5 grams per cubic meter. It is for this reason that many people, including scientists often say "warm air can hold more water than cold air." As Dr. Craig Bohren points out in his Meteorology classes and in the best-written atmospheric science book, Clouds in a Glass of Beer, air is not a sponge and does not hold water vapor. Instead water vapor molecules exist alongside air molecules. If that air with a temperature of 0°C contains 4 grams of water vapor per cubic meter, then this air has a relative humidity of approximately 80%.
Cloud Types
The three basic cloud types are: cirrus, stratus, and cumulus. Almost every cloud can be roughly classified by these groupings yet many more specific cloud names are used. Most names simply place prefixes and suffixes on the three basic cloud types. For instance altostratus indicates intermediate to high-level stratus clouds (using the prefix "alto" meaning high). Likewise, cumulonimbus appends the suffix "nimbus" to cumulus and indicates rain. Taken in the form "nimbo" as a prefix, nimbostratus refers to a persistent rain (or snow) falling from a low stratus cloud.
Cirrus Clouds
Cirrus clouds exist at the highest altitudes (20,000 ft and above) and are composed of ice crystals and smaller snow particles. They can be so thin they're barely perceptible or they can be a thousand or more feet thick. When very thin, they can be responsible for a myriad of optical phenomenon including multiple halos, perhelia, coronas, iridescence (irisation), and a variety of colorful arches.
Cirrus clouds often look like filaments or feathers. If they start producing precipitation size particles, they can form fallstreaks. Cirrus clouds can also be created by airplanes because they emit water vapor (along with most of the same pollutants as cars), which condenses into tiny ice crystals thus producing contrails. Over time, these ice crystals can spread out and be carried great distances by the wind because they are so small and fall so slowly. Other types of cirrus clouds include billows and mare's tails.
Stratus Clouds
Stratus clouds are found in the lower and middle portions of the atmosphere (ground to 30,000 feet perhaps). They are well known to the folks in San Francisco (and anyone living near coasts) where a persistent stratus cloud deck occupies the sky. When a stratus cloud is in contact with the ground, it is usually referred to as fog.
Stratus clouds tend to bore people, as well as depress them, but, for someone who works on aircraft icing, they are quite interesting. They are characterized by a uniform looking cloud base and top. They are primarily composed of water drops (but not necessarily) and can produce a pilot's bow, or glory.
Stratus clouds generally form under relatively stable atmospheric stratification where temperature weakly decreases with height. Also, when the wind blows moist air up against mountains (referred to as "upslope"), often a stratus cloud results. Though gloomy and perhaps raining or snowing at low elevations, you can often hike up above the stratus and bask in sunshine above the clouds as well as view what appears to be an ocean below.
Cumulus Clouds
In contrast with stratus clouds, cumulus clouds (from the Latin word 'heap') are typically found where the atmosphere is relatively unstable (temperature sharply decreases with height). Upper portions of cumulus tend to resemble domes or towers. Most summertime severe weather occurs by way of cumulus clouds including lightning, hail, and tornadoes.
Cumulus clouds often look like cottonballs or cauliflower. Or for more imaginative cloud objects, Richard Carlson maintains the Cool Clouds for Kids of All Ages web site with imaginative cloud forms like bunnies, dogs, saucers, etc.
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Suggested reading & related info:
Book: Clouds in a Glass of Beer: Simple Experiments in Atmospheric Physics
Book: The Invention of Clouds: How an Amateur Meteorologist Forged the Language of the Skies
Book: The Man Who Named the Clouds
Magazine: Weatherwise
Web: Bad Meteorology by Allistair B. Fraser.
Web: The Cloud Appreciation Society by Gavin Pretor-Pinney.
Web: Cool Clouds for Kids of All Ages by Richard Carlson.