Savoy cabbages are immediately recognizable on the shelves of your supermarket. They're shaped into a tight, round head, like conventional green or red cabbages, but the leaves have the distinctively wrinkled appearance of Napa cabbage leaves. Savoy varieties are milder-flavored than regular green cabbage, but the two can be used interchangeably in recipes.
Conventional green cabbage can be used as a substitute for savoy cabbage in most dishes. Both varieties produce tight heads, without the oversized ribs found on Asian cabbages. The leaves can be used whole as wrappers for rice or meat-based fillings, or the head can be cored and cut into wedges for boiling or steaming. When shredded for slaw, green cabbage's stronger flavor is noticeable but not overbearing. If you are stir-frying your cabbage, the difference in flavor can be minimized by blanching your shredded green cabbage for one minute in boiling water, then draining it and adding it to the wok.
Napa cabbage is similar to savoy cabbage in many respects, but it grows in loose, open heads of straight leaves, rather than a tight, round head. Its ribs are broad, juicy and mild-tasting; they are often cooked separately from the leafy portion. The leaves themselves have a textured, wrinkly appearance, much like savoy leaves. Large Napa leaves can be used like savoy leaves for wrapping grain or meat-based fillings, and it can also be shredded for stir-frying or slaw. Napa cabbage is milder than savoy; it has a slightly more delicate texture as well.
Many Asian greens belong to the cabbage family, most of which can be steamed or stir-fried. They're functionally equivalent to cabbage, but their darker green leaves and different textures make them indifferent substitutes for savoy cabbage. Ironically, one that does substitute well is not a cabbage at all, but a lettuce. Chinese lettuce forms long, open leafy heads, rather like romaine lettuce or Napa cabbage. It resembles Napa, but the leaves are longer and thinner. It's bitter when eaten raw, but when it is stir-fried, boiled or steamed, it is interchangeable with Napa or savoy cabbage.
Kohlrabi is one of the odder vegetables in the cabbage family. It consists of a swollen, spherical stem with just a few vestigial leaves distributed around the outside. The skin can be eaten when the kohlrabi is small, but -- like broccoli skin -- it quickly becomes woody and tough as the vegetable matures. Kohlrabi isn't a suitable substitute for savoy cabbage in dishes that use the leaves whole, but when shredded for slaw or stir-frying, its mild, sweet flavor makes it a good option.