I've had this one hanging around for days and since I'm on a roll just now, and just got a question about D, here goes. I'm not going to dig up the many past posts, but you can find most of them here, or simply search vitamin d to the right.
The short version is that vitamin D is crucial for a host of processes and modern life has come to the point of shielding humans from receiving the vitamin as nature and evolution intended. The things we already know about are clothing, shelter, working indoors and sunscreen that keep us from the D we need. But here's another: soap. Yep, all you clean freaks: you're washing your vitamin D off before it gets absorbed.
It was an interesting discovery for me, as it has been a very long time since I've put soap to anything but hair, face, armpits and groin. I never use lotions or creams and I have wonderfully soft skin. Maybe that's one reason for my high levels of D, along with supplementation, of course (now 4k IU per day instead of 6, since I get sun about 4 times per week).
So, here's Dr. Mercola to explain, with a video and a write up.
I must say that I disagree with his hierarchy of the most preferred way to get D. I think you need to get tested and that your 25-hydroxy vitamin D levels ought to be above 60 ng/ml. Unless you're living at a low latitude, I don't see how you're going to get that from sun exposure, at least year round. Tanning panels or beds are very expensive, and they strike me as rather like the treadmill in terms of eventual boring drudgery and something that ends up sitting in your house just taking up space. Supplementation is inexpensive, takes no time, is safe, and in the gelcap form is proven to get levels of D where they ought to be.