This vegetable goes by many names: luffa, angled luffa, Chinese okra, and silk squash. Dark green with ridges that run every quarter inch or so through the length of the squash, Chinese okra does not have much in common with the little finger-length squash we know as okra in the States.
I prefer the name "luffa," an apt reminder of the fact that if the squash is brought to maturity and dried, it can actually be used as a sponge (spelled alternatively as loofah).
Younger, immature luffa are wonderful to eat. In their prime eating stage, the taste and texture resembles zucchini, though luffa are far better at sopping up liquid, as the sponge reference suggests.
You can see from the cross-section here that the squash is foamy once you cut past the dark skin. Steamed or simmered, sections of luffa will surprise you with how much liquid is being held in the interior flesh. It may be the most succulent squash I've cooked, come to think of it.
You'll find luffa at Chinese and Southeastern Asian markets. Choose firm, unblemished luffa that are around 10 inches long with a soft, almost velvety skin that yields slightly when squeezed. Older luffa will have rougher, harder skin.
While luffa isn't one of the most attractive vegetables at the market, what it lacks in looks it makes up for in utility. I find it most enjoyable when steamed because the flesh retains its juiciness and delicate flavor while sopping up the broth or sauce when cooking. Steamed with stock, a plate of luffa can be finished with a drizzle of sesame oil and soy sauce, or add browned garlic and slices of chili pepper for additional flavor.