From Academic Kids
Carding is the processing of brushing raw or washed fibers to prepare them as textiles. A large variety of fibers can be carded, anything from dog hair, to llama, to soy silk (a fiber made from soy beans). Cotton, wool and bast are probably the most common fibers to be carded. Not all fibers are carded. Flax, for example, is threshed not carded.
Carding can also be used to create mixes of different fibers, or different colors. Some hand-spinners have a small drum carder at home especially for the purpose of mixing together the different colored fiber that they buy already carded. Some drum carders even come with directions on how to best card two colors at once.
The two main ways to card fibers are by hand, and by machine.
Hand carders, somewhat like dog brushes, and are used two at a time, brushing the wool between the two until all the fibers in a bunch are going in the same direction. Carding is an activity done most conveniently sitting down. Depending on how dirty the fiber being carded is, sometimes this activity is best done outside, or over a drop-cloth. One carder per hand, and the carder in the non-dominant hand (left for most people) gets put on the leg. A small amount of fiber gets put on the carder located on the leg. Then one uses the carder in the other hand, and pulls it through the other, catching some, but not too many, of the fibers. If one catches too many fibers it becomes too hard to pull the carders apart. If one catches too few, carding will take forever and one may not get all of the fibers. After a while, you move the fiber to the other carder, and do the same sort of motion as before, this time transferring fibers from the carder in the air to the one on your leg. When satisfied that all the fibers are in line, the person carding cleans off the carders and rolls up the fibers into a neat rolag (http://www.hants.org.uk/HantsGuildWSD/images/weaving/rolag2010.gif), the end result.
Hand carders come in a wide variety of sizes, from ones two by two inches to ones four by eight inches. The small ones are called flick carders, and are used just to flick the ends of a lock of hair, or to tease out some strands for spinning off of. The density of the teeth, and the shape of the carders also varies. For finely carded rolags, one uses carders with more teeth. The type of fiber, its length, weight and characteristics, can also determine how many teeth are wanted per inch on the carders. Hand carders can be either flat backed or curved- this is a matter of personal preference.
Machine carding is done on a device called a drum carder. These devices vary in size from the one that easily fits on the kitchen table, to the carder that takes up a full room  (http://pictures.galenfrysinger.com/new_england/sturbridge47.jpg). The carders used currently in woolen mills differ very little from machines used twenty to fifty years ago, and in some cases the machines are from that era. For wool, and wool-like fibers (such as llama, alpaca, goat, etc.) fibers are fed onto a series of rollers. depending on the size of the carder, the number of rollers differs. The ones that fit on the kitchen table typically have two drums, or rollers. One is small, and used to catch the fibers and feed them in. The other drum takes the fibers from the first drum, and, in the process of transferring them form one drum to another, the fibers are straightened out and told to be orderly. The picture above is a small drum carder. A carder that takes up a full room works very similarly, the main difference being that the fiber goes through many more drums, which normally get finer as the fiber progresses.
When the fiber comes off the drum, it is in the form of a bat, or a flat, orderly mass of fibers. If a small drum carder is being used, the bat is the length of the circumference of the big drum, and is often the finished product. A big drum carder though, will then take that bat and turn it into rovings, by stretching it thinner and thinner, until it is the desired thickness (often rovings are the thickness of a wrist). (A rolag differs from a roving ( (http://scrubberbum.typepad.com/moth_heaven/images/roving.JPG) or  (http://altamistalpacas.com/images/Roving.jpg)) because it is not a continuous strand, and because the fibers end up going across instead of along the strand.) Cotton fibers are fed into the machine, picked up and brushed onto flats when carded.
This product (rovings and rolags) can be used for spinning.
Carding of wool can either be done "in the grease" or not, depending on the type of machine and on the spinner's preference. "In the grease" means that the lanolin that naturally comes with the wool has not been washed out, leaving the wool with a slightly greasy feel. The large drum carders do not tend to get along well with lanolin, so most woolen mills wash the wool before carding. Hand carders (and small drum carders too, though the directions may not recommend it) can be used to card lanolin rich wool. A major benefit of working with the lanolin still in the wool is that it leaves you with soft hands.
- Directions on hand-carding, complete with numerous pictures. (http://www.worldknit.com/howto/howtospin/sfiberprep/card.html)
- How To Use Hand Carders (http://www.joyofhandspinning.com/hand-carding.html)