Belial is correct. The difference is in the size of the oats. The instant groates have been cut into smaller/more pieces, and they are much thinner. Therefore, they require less cooking time. First, there is regular oatmeal (15 minutes of cooking), Quick oats (~5 minutes of heating), and then there is Instant oats (which need around 1-1/2 to 2 minutes of cooking). The instant oats have additives such as starch, defatted wheat germ, and salt. They are also steamed so that it pregelatinizes the the starch so that it can be rehydrated with H2O faster (less cooking time). The only advantage of the regular oats is that they will not lose their chewy texture upon standing.
The difference between grits and oatmeal is that they are from 2 different kernels. Grits are from the corn kernel, and oatmeal is from the oat kernel. If I remember right, grits have been degermed. Degerming removes the fat and bran away from the cornmeal before it is grounded. This raises the ratio of starch in the product. Degerming is necessary because the high fat content of the germ turns rancid when stored for too long. Oatmeal differs in that the whole kernel is eaten, that is, the germ is not separated from the endosperm during milling. Oats are steamed so that they inactivate the lipase content of the groats, which will cause the fat to turn rancid if not.
As long as you are not buying that sugar infested instant oatmeal, you should not worry too much.
MR. BMJ