Carding partially aligns the fibers and forms them into a thin web that is brought together as a soft, very weak rope of fibers called a carded sliver. Carding is when short wire teeth remove trash and neps (entangled clumps of short, underdeveloped fibers).
Combing is used if long-staple fibers are to be spun. It produces a yarn that is superior to a carded yarn in smoothness, fineness, evenness, and strength. Combing aligns fibers in a parallel arrangement & also removes short fibers so fibers in the combed sliver will be more uniform in length.