When we talk about the sun's rays we mostly talk about its UVA and UVB rays.
UVB rays are stronger in Summer and are the rays that can cause skin reddening or sunburn.
But UVA rays are just as strong in Winter as they are in Summer. It's these rays that penetrate the skin deeper and which have long been known to play a major part in skin aging. And, more recently, scientists have discovered that UVA rays can also cause significant damage in areas of the epidermis (outermost layer of the skin) where most skin cancers occur.
Where strong Summer UVB rays can lead to reddening of the skin and sunburn, it's the UVA rays which create sun-tan. And whether your suntan is a result of being outdoors or from a sunbed in a salon, suntan is damage to the DNA of your skin; the skin darkens in an imperfect attempt to prevent further DNA damage and these imperfections (or mutations) can lead to skin cancer. Damage caused by UVA rays is irreversible.
When you apply sunscreen every day, all year round, you are not only protecting your skin from burning in summer, but you're protecting it from premature aging caused by UVA rays and reducing the chance of developing skin cancer.