I think the term "Bulgarian yogurt" has come to mean any yogurt that has been cultured with the Lactobacillus Bulgaricus strain. As it is most yogurt (thermophilic yogurt) is made by heating milk before adding the culture; the standard recipe calling for the milk to be heated to 185F and then cooled down to 115F. Supposedly it affects the protein strands or something. At any rate, you can use raw, unpasteurized milk to make yogurt but the problem there is that the bacteria already existing in the milk will compete with the culture that you add, so heating the milk to 185F helps with this issue also.