There are three different types of UV radiation - UVA, UVB and UVC - which are classified based on length of their waves.
UVC, wuth the shortest wavelenghts, is completely absorbed by the ozone in the Earth's atmosphere and is therefore of little concern. Both UVA and UVB can break through the ozone layer and reach the ground and are responsible for skin damage and skin cancer. UVA (with the longest wavelenghts) can penetrate deeply and damage the skin's lower layers. UVA is responsible for sunburn, wrinkles, freckles, suntan, outer layer and cause sunburn and skin cancer.
A helpful rule of thumb is: "A (UVA) is for aging, B (UVB) is for burning."
Available at: www.sunsafecolorado.org/school/uvradiation/uv_uv.shtml