Pak Choy or Bok Choy is a deep green leaved member of the Brassica family. It has thick creamy white stems that are succulent, with deeply crinkled, shiny leaves that have a flavor reminiscent of Swiss chard. It is a favorite of Chinese cuisine, and is grown extensively in Asia. It is found fresh or dried. 

Here are some links to Pak Choy sites:

http://www.gardenguides.com/articles/bokchoy.htm

http://www.ornamentaledibles.com/asiancabbage.html