The semolina found in packets here is coarsely ground wheat
Made into a pudding it is horrid (well, some people like it...), but it can be added to cakes with terrific results. It gives a wonderfully crumbly, delicate texture, especially in almond cakes, where it helps to counteract the denseness that ground almonds and lots of egg-yolks can give. You could use cornmeal or polenta for this, but the corn flavour can be overwhelming and you don't necessarily want that. (Although polenta cakes and biscuits can be delicious.)
It's also really good when you roll out the dough for pizza, giving a crisp crust when it bakes. Nigella Lawson also uses a small quantity in place of breadcrumbs when she makes meatballs. (I haven't tried it, but I'm sure it's a good substitution.) Maison Blanc sells a beautiful white 'semoule' loaf that is well worth trying.