When a recipe calls for “Chinese cabbage”, what exactly are you supposed to buy and use for the dish?

Chinese cabbage (Brassica rapa) is classified into two main groups: Pekinesis (napa cabbage, above) and Chinensis (bok choy). There are several hybrids in each group.
Pekinensis (napa cabbage)
We call it pechay baguio in the Philippines. In English-speaking countries, its most common name is wombok.
Napa cabbage is the only Chinese cabbage that grows a head. The leaves are light green (some cultivars have darker leaves), the stalks are white and white veins spread through the leaves.
Both stalk and leaves are very tender that they can be eaten raw. The flavor is quite delicate — bland, in fact — which makes it the ideal vegetable for making baechu-kimchi (literally, cabbage kimchi), the most common kind of kimchi.
For home cooking, cut off the root end of the napa cabbage to separate the leaves. Whether or not to separate the white stalks from the leaves depends on the dish you’re making.
Chinensis (bok choy)
Chinese cabbages falling under the Chinensis group do not form heads. Commercially, they are sold under various names including bok choy, pak choy, pei tsai and choy sum. Some hybrids have white stalks (like the Filipino pechay); others have light green stalks (like Taiwanese bok choy).

Unlike napa cabbage, bok choy has a more pronounced flavor. Occasionally, the leaves can be a little bitter. The stalks are more fibrous and are, therefore, ideal for stir fries.
To use Chinese cabbage under this group, it is best to separate the leaves from the stalks. Add the stalks to the pan first and give them a head start to allow them to soften before adding the leaves which require a much shorter cooking time.
Recipes with Chinese cabbage
Updated from a post published in September 19, 2009