If you are Caucasian and you don't have a tan, then the cells in your skin are not protected from the sun's ultraviolet radiation. You are therefore an easy target for sunburn if you spend too much time in the sun. As anyone who has a sunburn knows, sunburn leaves your skin red and extremely painful. In severe cases the skin forms blisters.
When you get sunburn, your skin is actually damaged by UV radiation and your body is responding to the damage. Here is a specific medical description of what is happening from this article:
Sunburn is a popular term applied to the marked erythema and pain that commonly follows injudicious sun exposure. A sunburn is really a delayed ultraviolet B-induced erythema caused by an increase in blood flow to the affected skin that begins about 4 hours and peaks between 8-24 hours following exposure [23, 30, 31]. The underlying cause of this vascular reaction is direct and indirect damage to specific cellular targets from photochemical reactions and the generation of reactive oxygen species . Damage to DNA, and the activation of several inflammatory pathways, particularly involving prostaglandins [27, 33-38], are thought to trigger this reaction, ultimately leading to vasodilation and edema. Biologic response modifiers released by both keratinocytes and lymphocytes also play a role [39-55]. The development of erythema therefore implies that enough ultraviolet damage has occurred that inflammatory pathways have been activated. Erythema is probably best thought of as a total failure of sun protection, and is a marker for severe UV damage.
Several lines of evidence suggest a relationship between erythema and DNA damage. There is rough correlation between pyrimidine dimer yield and susceptibility to erythema with sun exposure . Wavelengths that are the most efficient at producing erythema are also the most efficient at producing pyrimidine dimers . From a scientific point of view, a sunburn can be viewed as a marker for a substantial ultraviolet over-exposure that has clinical implications for skin cancer risk. It is now appreciated that there is a linkage between a history of repeated, severe sunburn and increased risk for melanoma [1, 57-62] and non-melanoma skin cancer [63-65].
Translating, what this quote says is that when you get a sunburn, what you are really getting is cellular damage from ultraviolet radiation. The body responds to the damage with increased bloodflow to the capillary bed of the dermis in order to bring in cells to repair the damage. The extra blood in the capillaries causes the redness -- if you press on sunburned skin it will turn white and then return to red as the capillaries refill.