Same stuff, different size. After the grinding of the corn, it is passed through graded sifters. The coarsest particles end up as grits. Meal is slightly finer.
Notably, you can use cornmeal to make grits, and you can use grits in lieu of cornmeal, but you’ll find that if you use cornmeal you’ll end up with very smooth & fine grits, and vice versa—grits are a bit crunchier.
Hominy grits are mader from corn that has been subjected to a process called nixtamalization. They’re different from plain grits, but I suspect that people are imprecise in how they use the terms.
Ahh – this would explain why my cornmeal-lime-chile crusted tofu was REALLY crunchy last week….I accidentally used grits instead of meal! Still good though – just a weird texture.