Mustard seeds are the small round seeds that come from several different types of mustard plants. The seeds are approximately 1-2 mm in diameter. The seeds (also called fruit pods) come from the mustard plant in the Brassica family. Some of the other members in this family include broccoli, Brussels sprouts and cabbage. Mustard seeds are harvested from three different plants: white mustard (better known as yellow mustard from Brassica alba), brown mustard (Brassica juncea) and this very hard to find black mustard (Brassica nigra).