Vitamin D_3, is a fat soluble vitamin produced by our systems and found in a few animal sources.

Vitamin D₂ is not produced by our systems. It is derived from fungal and plant sources.

Vitamin D₃ is needed for strong bones. It also helps boost the immune system and is important for overall good health.

When ultraviolet rays from the sun strike the skin it triggers vitamin D synthesis. For maximum benefit, the sun you get needs to be direct, not through windows or cloud cover, both of which cut effectiveness by over 50%.

For people with normal skin twenty to thirty minutes a day of direct sun is great. Those with sensitive skin should limit direct exposure to between five and ten minutes a day. Do not over-do sun exposure.

Vitamin D is also present in a few foods. These include salmon, tuna, eggs and cod liver oil.

The recomended daily allowance of Vitamin D is 200 international units (IU) up to age 50 and 400 IU after that, but researchers doubt this is enough. They believe we need at least 800 to 1000 IU per day, especially breast fed babies, older adults and dark skinned people and those who do not get much sun.

Vitamin D can be toxic when taken in large doses in suppliment form. However pigging out on natural foods does not cause toxicity.

Sunscreens can limit your vitimin D production. Allow yourself at least 20 to 30 minuts a day without using a sunscreen. After that do use enough sunscreen to avoid overexposure to ultraviolet damage. Darker skin to lighter, we need to use SPF 15 up to SPF 45.

Many people apply sunscren when they go out in the sun and don't think about how they are sweating it off. To really protect your skin sunscreen needs to be reapplied at least once every hour or so.
Wickopedia detailed overview of Vitamin D