It's the sun's UVA rays – the longest rays in the spectrum, which penetrate deepest into the skin. And it's these which cause damage at a cellular level, making them responsible for the signs of ageing in the form of wrinkles and pigmentation.
UVA is present all year round, even in winter. UVB rays are shorter than UVA rays, and cause the burn or tan effect in the skin. Both UVA and UVB rays can cause cancer, so wearing a broad spectrum sunscreen with antioxidants is absolutely essential.
UVC radiation is potentially the most damaging, but it's absorbed by oxygen and ozone in the stratosphere and never reaches the Earth’s surface.