Grits means different things to different people. Here, it's hominy grits, and here, hominy means corn that has been treated with lime, which is a beneficial thing in terms of nutrients. But other places, grits means plain milled corn, even though it may be called "hominy grits," because hominy also gets used some places with "grits" without meaning the corn has been limed.
Probably the most commonly available grits in the U.S. is Quaker brand, but more and more, only the quick and instant appear on store shelves. Most people who depend on their grocery alone, may find only "quick" grits, the five-minute type. And they say they use "hominy corn," which I take to be limed. But that's Quaker.
"Quick grits" may also mean a finer grind for 15-minute cooking time, while course grits may take 50 to 90 minutes. The course grits are the old style traditional.
Anson Mills, probably the best source of such things, grind grits from a variety of corns, and their products page is worth reading. Note the real distinctions among grits, cornmeal, and polenta. Eric, they don't have a local retailer, so it's online ordering, if you want any of this.
Their offerings are also a source for varying preferences in polenta, since they offer different grinds. And their "polenta" is appropriately made with flint corn.
Note in their recipes, one for "hominy" and their offering of culinary lime.
Anson Mills Recipes