What it is
Milk fermented by live bacteria — one of the oldest preserved foods there is. Plain and whole-milk is food; the fruit-on-the-bottom tubs are dessert.
Why your biology objects
The cultures convert lactose to lactic acid and supply their own lactase, which is why many lactose-intolerant people tolerate yogurt; straining (Greek) roughly doubles the protein. The footnote is added sugar and the low-fat versions that replace the fat with starch and gums.