Mustard is the yellow sauce people commonly use to spice up their hotdogs. Whether it's ballpark mustard or fancy Grey Poupon, it's a condiment made from the ground seeds of the mustard plant.
Mustard comes in several different forms, including the bright yellow kind in a packet at your favorite fast food restaurant and the brownish, slightly coarse mustard that comes in a fancy jar labeled "Dijon." All of these mustards are made from mustard seeds and spices, often mixed with lemon juice and salt. The plant that grows these seeds also produces edible leaves called "mustard greens."