Skin cells are alive, but hair cells are dead. A suntan develops when the skin is exposed to ultraviolet light. Chemical changes (oxidation) in the skin cells darken the melanin already present and stimulate the production of even more melanin in an attempt to protect the skin from further damage. Continued ultraviolet exposure causes further damage, including destruction of melanin, but the melanin can be replaced as long as the cell hasn't been seriously damaged.

Melanin is also present in hair, and it too undergoes oxidation under ultraviolet light. However, destruction of melanin is permanent in dead cells, and so the hair becomes lighter in color with prolonged exposure to sunlight.
Source(s):
http://en.wikipedia.org/wiki/Sun_tanning#Cause_and_effect
http://www.thetech.org/genetics/ask.php?id=180

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