Cause and effect

Darkening of the skin is caused by an increased release of the pigment melanin into the skin's cells after exposure to ultraviolet radiation. Melanin is produced by cells called melanocytes and protects the body from absorbing an excess of solar radiation, which can be harmful. Depending on genetics, some people can darken quickly and deeply whereas others do not darken much at all.

The ultraviolet frequencies responsible for tanning are often divided into the UVA (315 to 400nm wavelength) and UVB (280 to 315nm wavelength) ranges. UVB have higher energy than UVA waves and are therefore more damaging and more carcinogenic.

UVB
triggers creation and secretion of new melanin into the skin
is thought to cause the formation of moles and some types of skin cancer (but not melanoma)
causes skin aging (but at a far slower rate than UVA.)
stimulates the production of Vitamin D
is more likely to cause a sunburn than UVA as a result of overexposure, however moderate exposure can be healthy
is almost completely blocked by virtually all sunscreens

UVA
causes release of preexisting melanin from the melanocytes
causes the melanin to combine with oxygen (oxidize), which creates the actual tan color in the skin
seems to cause cancer less than UVB, but causes melanoma, a far more dangerous type of skin cancer than other types
is not blocked by many sunscreens but is blocked to some degree by clothing
is present more uniformly throughout the day, and throughout the seasons than UVB