The word mustard is used to refer to several species of plants. These plants belong to the genera Brassica or Sinapis. They have tiny seeds. These seeds can be used as a condiment. When they are first ground, and then mixed with water, vinegar and other ingredients, a condiment called mustard can be made. The seeds can also be pressed, to give a kind of oil called mustard oil. Some kinds of mustard plants have edible leaves. These leaves are known as mustard greens.
Mild white mustard (Sinapis hirta) grows wild in North Africa, the Middle East and Mediterranean Europe and has spread farther by long cultivation; brown or Indian mustard (B. juncea), originally from the foothills of the Himalaya, is grown commercially in the UK, Canada and the US; black mustard (B. nigra) in Argentina, Chile, the US and some European countries.
There has been recent research into varieties of mustards that have a high oil content for use in the production of biodiesel, a renewable liquid fuel similar to diesel fuel. The biodiesel made from mustard oil has good cold flow properties and cetane ratings. The leftover meal after pressing out the oil has also been found to be an effective pesticide.  Archived 2004-11-17 at the Wayback Machine