How is Vitamin D3 different from regular Vitamin D?
http://vitamind3.info/p145964-how-is-vitamin-d-different-from.cfm 
January 15, 2010
Vitamin D3 is manufactured by the body when human skin is exposed to direct sunlight. It can also be obtained through certain foods, as well as supplements. The main difference is in D3's superior bioavailability, which lends to better absorption of the nutrient and enhanced health benefits directly associated with absorption factor and the fact that D3 is a more naturally occurring form of the nutrient.
According to Nurse Practitioner Pam Egan, Vitamin D3 is "not to be confused with inactive Vitamin D2."
Two Major Types of Vitamin D
Although there are a total of five different forms of Vitamin D, only two are commonly used in supplements.
   1. Ergocalciferol (Vitamin D2) - A synthetic form of Vitamin D. This is the form found in most nutritional supplements. D2 is substantially less bioavailable than D3.
   2. Cholecalciferol (Vitamin D3) - The naturally occurring form of the D Vitamin. This is the same Vitamin D manufactured by the human body when skin makes contact with direct sunlight. Cholecalciferol is five-times (or 500%) more bioavailable than Ergocalciferol, which means that D3 is converted into a usable form by the body at a rate five times faster than is the case with D2.
Another drawback with Vitamin D2 is that it has a much shorter shelflife than does D3, and its metabolites do not bind well with proteins, rendering it that much less effective than the alternative.
Many recent studies have suggested that Vitamin D2 should no longer be classified as a nutrient appropriate for supplementation and/or fortification in foods. Despite this, it continues to be the most common form of Vitamin D available in supplement form on the market today.
In conclusion, based on all of that documented above, it is clear that Vitamin D3 is a far superior form of Vitamin D than is D2, and anyone who takes their health seriously should strongly consider double-checking to make sure they're getting the good stuff (Cholecalciferol) as opposed to the inferior Ergocalciferol when shopping for Vitamin D supplements.