The processing of the two yogurts is different. Milk contains two proteins: casein and whey. When making yogurt, the casein stays more solid, the whey more liquid. Low-fat plain yogurt contains both, and so is a "loose" solid. Greek-style yogurt is thicker and more solid because it is strained and the liquid whey protein is removed as part of the processing, giving it a richer and creamier texture. Nutritionally, Greek-style yogurt is higher in protein and lower in lactose than regular low-fat plain yogurt.
Susan Kleiner, PhD, RD, is an international columnist and a speaker on the subjects of nutrition, sports and fitness.