Dandruff is the shedding of dead skin cells from the scalp. As skin cells die, a small amount of flaking is normal; about 487,000 cells/cm2 get released normally after detergent treatment. Some people, however, experience an unusually large amount of flaking either chronically or as a result of certain triggers, up to 800,000 cells/cm2, which can also be accompanied by redness and irritation. (source)