Well milk is made up of casein and whey. Casein is what curdles and is
used to make cheese and yogurt, while whey is the watery substance left
when the milk curdles. So what is Greek yogurt? Greek yogurt is
essentially the same as regular yogurt with the addition of one critical
step – straining! At the very end, the yogurt is strained to remove the
excess watery whey, creating a thicker, more protein-dense product. So
while we call this variation of yogurt “Greek yogurt” in the U.S., you
may see it called “strained yogurt” in other areas of the world.