Vitamin D is a fat soluble vitamin. It can also be called as a steroid vitamin. It is a vital nutrient because it aids the absorption of calcium and phosphorous. Sunlight is the best source of vitamin D. People who are exposed to normal quantities of sunlight get enough from it and so there is no need for any additional supplements. Sunlight promotes synthesis of vitamin D in the skin. Basically there are five forms of vitamin D, vitamin D1, D2, D3, D4, and D5.
Functions:  

The D vitamin is essential for enhancing immunity and fight disorders like common cold or other health infections.

It enhances the capacity of absorption of calcium and phosphorus in human body, thus improves bone health.

Vitamin D promotes brain’s health.

It reduces the risk of multiple sclerosis and rheumatoid arthritis.

It lowers the risk of cancer.

It helps in generation of energy. 

In most cases our body is found to be deficient in vitamin D due to insufficient exposure to sunlight. In that case, it would be vital to depend on foods and food supplements that have vitamin D. Some common food items containing vitamin D are: salmon, cod liver oil, margarine, tuna, eggs etc.