What is
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.
  
Soured milk is a food product, distinguished from spoiled milk, and is a general term for milk that has acquired a tart taste, either through the addition of an acid, such as lemon juice or vinegar, or through bacterial fermentation.