is a brand-name for oil clay, which, (by means of guessing based on the name) is clay that has been impregnated with oil. If you want to split hairs, I don't think sculpey
count as oil clay.
School grade oil clay comes in a four pack of brightly, but somehow off-colored bars, red, blue, yellow, and gray (sometimes brown, too). These are very soft and will leave your hands colored after playing with it. The upside is that, after a few years, it doesn't stain as badly, and isn't nearly as slimy-soft. (By that time, it's all probably mixed into a greenish-gray lump, too.)
If you go to an art store, you can buy the stuff without all that hideous coloring. You have the choice of ivory, gray, or green, colorwise; and extra-soft to extra-firm texturewise, in about five steps, I think. I usually get the 2nd-hardest, as it seems to have the best resolution while still being somewhat malleable.