There are two ways to prepare fibres for spinning: combing and carding. Combed wool, silk, cotton, or flax is called top, while carded wool, silk, cotton or flax is usually called roving.
If you grew up in the United States or Canada, you probably saw women carding wool at pioneer villages or in old television shows about frontier life. A hand carder looks something like a dog brush: its face is wide and flat, about the size of a person's hand, with one surface filled with short, stiff wire bristles. The number of bristles per square inch is called the point: high point is good for fine fibres like cotton and mohair, while low point is better for coarser wool and flax. Some hand carders are slightly convex, making it easier to roll them against one another. The tips of the bristles should bend very slightly in the direction of the handles.
In the past, women would use a pair of hand carders to remove knots from fleece, and to fluff out and align the fibres in preparation for spinning. It is still possible to buy hand carders from specialty craft shops, and some of them are lovingly crafted of beautiful woods.
A drum carder is a contraption made up of a base, covered with bristles, and a large cylinder, also covered with bristles. The cylinder can be turned with a crank -- or, if you've got a really newfangled sort, with an electric motor. A drum carder can process a large amount of wool with a minimum of effort, but boy, is it ever a bear to clean.
Carded wool can be made into round blobs called batts or ropelike lengths of roving. Handspinners who do not want to go through the bother of shearing a sheep, washing the fleece, then carding the wool all by themselves can buy bags of roving and start spinning immediately. Some people even knit roving without spinning it first. The resulting fabric is not as durable as it would be if it were made from real yarn, but it can be nice for mittens or slippers.