Vitamin D is an umbrella term for several forms of the vitamin, and vitamin D3 is one of those forms, according to the Mayo Clinic. Vitamin D3 is made by the skin from sunlight exposure. Food sources of vitamin D include fortified milk, eggs, fish and cod liver oil.
Vitamin D3, or cholecalciferol, supplements are used to treat vitamin D deficiency, the Mayo Clinic explains. The supplements are available both as a doctor’s prescription or over-the-counter product in tablet, capsule, wafer and solution dosage forms. People with limited sun exposure, obese individuals and exclusively breastfed babies are at high risk of having vitamin D deficiency, which may cause weakening of bones and muscles.