Ultra Violet Radiation 
What gives you your tan (as well as other benefits), and can also do you damage, is Ultra Violet radiation coming from the sun, whether it hits your skin directly or bounces first off some surface like the sea, sand, a white wall or even snow.
The types of UV rays
The sun gives energy in
the form of light waves of different lengths and intensity. The
sunlight we see is called visible light. Ultra Violet radiation is an
invisible part of the light given out by the sun. UV radiation is made
up of different bands:
UVC: The UVC rays have the shortest wavelength, the most energy and fortunately are stopped by our atmosphere, particularly the Ozone Layer, from reaching us.
UVB: The UVB rays are the most potent rays that reach the earth. UVB rays cause sunburn (Think B=Burning Rays) and can also lead to the development of skin cancers. SPF (sun protection factor) ratings measure a product's ability to block UVB rays.
UVA: The UVA rays, while having less energy, penetrate deepest into the skin. These rays cause ageing (Think A=Ageing). These are the least associated with skin cancer. However, if UVA rays are absorbed in high dosages they can cause, not only skin cancers, but also allergic and non-allergic sun-induced eruptions, darkening and pigmentation of the skin (tanning), and acceleration of the ageing process. After long-term exposure, the skin may appear dry, scaly, spotted, wrinkled and leathery.
How the rays of the sun affects the skin
The part of the skin that is affected directly by UV radiation is the "Epidermis", that is the outer layer of the skin.
The
epidermis is your protection from the world. It has two main layers,
the inner of which is living and the outer of which is dead. The dead
skin cells of the outer layer are what we can actually see, and they
are constantly flaking off and being replaced by new cells being pushed
outward. Most UVB rays get no further but it is the ones that do that
can cause problems.
The living, inner layer is called the "Malpighian" layer which goes down to, at most, about 2mm deep.
The Malpighian layer creates the dead cells that we can see. It is in direct contact with the dermis, which feeds and supports it. It is here in the Malpighian layer, that the sun (particularly through UVA rays) affects the skin during tanning.
The Malpighian layer is itself layered, the bottom layer of which is the basal layer. This is the layer in contact with the dermis, where the nerves, blood vessels and the roots of hairs are located. The basal layer contains another group of cells called the melanocyte. These cells produce the melanin that provides the pigment which colours your skin. These melanocyte are cells in their own right and although there are about equal numbers of these cells in dark and light skin, it is how active they are and the amount of melanin they produce that makes the difference. Trouble starts when, from an overexposure to UV radiation, the DNA is changed and/or the cells start mutating.
