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Cloud Basics
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 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 (see below), 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. Nearly every cloud can be roughly classified by these groupings yet many more specific cloud names are used. Some cloud names simply place prefixes and suffixes on the three basic cloud types. For instance altostratus indicates intermediate to high-level status 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 (row 1)
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. Page 3 of this gallery contains a handful of these photos.
   Cirrus clouds often look like filaments or feathers. If they start producing precipitation size particles, they can form fallstreaks as in the middle photo of row 1 above. Cirrus can be created by airplanes which start out as contrails. [Maybe this is a good thing since cirrus clouds are thought to cool the earth in the global warming argument.] Other types of cirrus clouds include billows and mare's tails. An example of each of these is shown above.
 
Straus Clouds (row 2)
Stratus clouds are found in the lower and middle portions of the atmosphere (ground to 30,000 feet perhaps). Stratus clouds are well known to the folks in San Francisco (and anyone living near coasts) where a persistent stratus cloud deck occupies the sky. When the stratus cloud is in contact with the ground, we usually refer to this 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 (see photo above referenced by WxOpti03b_08). Stratus clouds generally form under relatively stable atmospheric stratification where temperature weakly decreases with height. 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 as well as view what appears to be an ocean below.
 
Cumulus Clouds (row 3)
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. Later parts of this gallery contains numerous example photos.
   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.
 
Suggested Reading:
   Bohren, C.F., 1987: Clouds in a Glass of Beer John Wiley & Sons Inc.
This is my single top-rated Atmospheric Science book for anyone wanting to learn more about basic physical Meteorology. Dr. Bohren is an inspiration to anyone writing about science and does so without unneeded, complex equations. I often refer to him as the "Robin Williams of teaching" (reference Dead Poets Society). He was a mentor as well as a catalyst for my foray into photographing weather phenomenon.