hey there, okay so D2 is ergocalciferol, a plant form. your body has to turn it into D3.
D3 is cholecalciferol, the animal form. your body can use this as is. it's the form your own body makes from 7-dehydrocholesterol, when it's irradiated by the right intensity of UVB light. (seasonally variable depending on your distance from the equator.. and the weather).
there are studies out there on the difference in terms of bioavailability and absorption. D3 is better. supplements are made from the fat on lambs wool. so next time you see a milk commercial touting its vitamin d content from 'the sunny meadows of new zealand', you'll know what they're on about