Oddly enough, Vitamin D is not really a vitamin. It is a hormone that is produced from cholesterol under the skin and comes in two different forms: D3 and D2.Vitamin D3, known as cholecalciferol, is made naturally in human skin when sunlight hits it. Vitamin D2, or ergocalciferol, is produced in plants and fungi when exposed to sunlight. D2 is less potent than D3 and does not raise your blood vitamin D level as efficiently as does Vitamin D3. D3 is what you generally find in supplement form.