Masa probably deserves its own post so let's just look at grits and polenta. Are they the same thing? As with so many items in life, the answer is yes, and no.
Not really satisfying, huh? OK, let's drill down a little further. Both are just ground corn, otherwise known as cornmeal. Boiling either will produce grits or polenta, depending on where your ancestors hail from.
But, and you knew this was coming, it can be rather more complicated than that. Different types of corn produce meal that varies quite a bit in its properties. Such as taste, storage, or even color. I've drawn on Anson Mills website for a great deal of the corn information that follows. They're a great place to get artisan grains and meals. And more information about them than you ever dreamed existed.
The corn that was exported to Italy in the 17th century was what is known as hard flint corn. This corn has an extremely hard kernel and takes multiple passes through a mill to produce a fine meal. This characteristic is also what gives polenta it's firm texture when cooked.
Corn in the American South however tended to be what is called soft dent corn. As you might expect from the name, the kernels in this corn were softer and could be ground in a single pass through a mill. The resulting product is a less uniform particle size and a less firm consistency when cooked.
So, where does that leave us? As a practical matter, unless your miller is willing to tell you precisely what kind of corn went into your meal, grits and polenta are interchangeable. If you do know, flint is for polenta and dent is for grits.
And what about white vs. yellow vs. ??? I think that's best left for another time.