The sun is the most natural source, but if you're dark skinned, or live in an area that's cloudy often, or protect your skin with pants, long sleeves and hats, you may not get enough, especially during the winter. Also, freshly-washed skin doesn't contain enough of the needed oils to help process the sunlight into Vitamin D.
Taking D with some fats/oils at mealtime helps absorbtion, since it's a fat-soluble vitamin. And vitamin D itself helps your body utilize calcium.
Here's a fairly comprehensive explanation: http://www.mayoclinic.com/health/vitamin-d/NS_patient-vit...
Also, NEWS FLASH: A letter to Science News about Vitamin D suggests that taking it as a supplement may help alleviate SAD, or Seasonal Affective Disorder, the weariness and depression that some people feel during the darker days of winter.