From the Kitagawa Seed Company:

This graceful vegetable with Chinese origins has spread throughout Asia and beyond, developing a wide range of varieties. The most typical Pak Choi features dark green leaves atop white spoon-shaped upright stems. Stems vary considerably in thickness and shape, and in some varieties they are green. One variety produces a rosette of dark green leaves close to the ground. There are specialty pak chois that have frilly leaves to light yellow-green color.

The slight mustardy flavor of Pak Choi makes it a delightful addition to stir-fries, soups, noodle and meat dishes, and salads, if the young leaves are used. In China, the coarser leaves are often pickled. Some Chinese cooks also dip the leaves in boiling water and hang them out to dry in the sun for several days. Drying enables this highly perishable vegetable to be stored for winter months. Asian cooks use the entire plant at many stages of development.

Photo credit: David Monniaux via a Creative Commons license