Our familiar yellow bananas represent the sweetest, softest type of plantain. Actually plantains come in many other gradations of size, color, and sweetness or non-sweetness. Oaxacan use them as a starchy vegetable in soups and stews and add them to certain moles.
Plantains, as we know them in the U.S., are tougher-skinned and usually larger than bananas. Different varieties may be green or yellow at the same stage of ripeness. Look for them in Latin American markets and when possible, try to choose ones that have turned partway from green (or yellow) to black. Sometimes they are used semi-ripe (only blackened in spots), sometimes at the highest stage of ripeness (black nearly all over), but seldom when they are green. If you have to buy them green you may need to plan ahead, as they take a few days to ripen.