What is the difference between grits and polenta?
True southern grits are made with ground hominy whereas polenta is simply ground cornmeal. The proper name for them is actually hominy grits. You can make "grits" out of untreated corn, but these are corn grits and not really found in southern US cuisine. Grits are typically a much coarser grind than polenta.

Hominy is corn that has been nixtamalized, which means soaked and cooked in an alkali solution, typically lye or limewater. This process converts the niacin present in corn into a form more soluble in the human body. Early European importers of corn from the USA dismissed this process as unnecessary, and as corn became a subsequent staple a pretty nasty disease called pellagra took hold caused by niacin deficiency.