Quark is a type of fresh dairy product, common for the cuisines of German-speaking countries made by warming soured milk until the desired degree of coagulation (denaturation, curdling) of milk proteins is met, and then strained.
Bulgarian Yogurt is a mildly sour tasting yogurt made from full fat fresh milk. The yogurt is made by using the strains of "Lactobacillus Bulgaricus"- a bacteria unique to Bulgaria.