Vitamin D3 is one of the many forms that vitamin D is found in. Vitamin D2 and D3 are the two most important forms of vitamin D for humans, according to MayoClinic.
The D2 form of vitamin D can be obtained through food sources like fish, eggs, fish oil and fortified foods. The D3 form on the other hand can only be created after skin is exposed to direct sunlight. One of the most important roles of vitamin D is to allow the body to efficiently absorb calcium, an important mineral for bone health. The main secondary role of vitamin D is to keep an appropriate level of phosphorus and calcium in blood.