While considered a variety of winter squash, acorn squash actually belongs to the same species as summer squash (zucchini, for example). Acorn squashes are relatively small. My specimen was maybe a pound and a half, and roasted it yeilded about 2 ½ cups of flesh. The most common varieties are dark green and may or may not have an orange blaze, but there are also golden and even white varieties. Look for squashes that have no soft spots and that are heavy for their size (indicating they’re not old and dried out). Like most winter squashes, these will store for months if kept in a cool, dry place.
Acorn squash is more subtly flavored than some of its fellows and has a ridged shape that can make it difficult to work with. It is commonly prepared by splitting it longitudinally, scooping out the seeds (a spoon is a very effective tool for this), and then roasting it in some manner. A favorite method is to slather the cut sides with a mixture of butter and brown sugar (or honey or maple syrup) and then roasting it cut-side-up in a 350-degree (F) oven till tender. My latest approach has been to roast it skin-side-up with nothing but a small bit of water to help steam it so that I can scoop out the pulp and use that in other dishes. I’ve recently attempted a savory acorn squash souffle, and it was tasty, but not great (if I can get it to be “great” I’ll post the recipe).