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.
Yogurt, in simple terms, is the bacterial fermentation of milk which is often sweetened or flavored. Lactobacillus & Streptococcus are the vital bacteria's for the yogurt's thick texture and consistency.