Mustard seeds are the proverbially small seeds of the various mustard plants. The seeds are about 1 mm in diameter, and may be colored from yellowish white to black. They are important spices in many regional cuisines. The seeds can come from three different plants: black mustard (B. nigra), brown Indian mustard (B. juncea), and white or yellow mustard (B. hirta/Sinapis alba).
In the Indian subcontinent they are often used whole, and are quickly fried in oil until they pop to impart a flavor to the oil.
The French have used mustard seeds as a spice since 800 AD, and it was amongst spices taken by the Spanish on explorations throughout the 1400s. Pope John XXII was particularly fond of mustard, and created a new position in the Vatican, 'grand moutardier du pape', or 'mustard maker to the pope'.
Mustard seeds generally take 3-10 days to germinate if placed under the proper conditions, which include a cold atmosphere and relatively moist soil.
Mustard grows well in temperate regions. Major producers of mustard seeds include Hungary, Great Britain, India, Canada (36%) and the United States. Brown and black mustard seeds return higher yields than their yellow counterparts. 
Mustard oil can be extracted from the seeds. The seeds, particularly the white ones, can also be ground into a flour, and mixed to a thick paste with a little water to make the condiment mustard. The ground mustard powder is usually mixed with ordinary flour to reduce the strength of the resulting condiment.
When initially mixed the sauce is mild in flavor, but it develops in time. Strong mustard has a very powerful (and painful) effect on the nasal membranes if eaten carelessly.
The whole seeds can be soaked in liquid before grinding to create whole grain mustard.
It is possible to buy prepared mustard in many places.
In the Parable of the Mustard Seed, Jesus compares the Kingdom of Heaven to a mustard seed. Although having some of the smallest seeds, the mustard plant grows to a large size, providing shelter for birds: Mark 4:31-32. The story has been interpreted to mean that grand things can grow from tiny actions.
Inspired by this parable, aristocrat Nikolaus Ludwig von Zinzendorf founded the Order of the Mustard Seed in Germany in 1715.  The aims of the order were to be true to Christ, kind to all people and to spread news of the Gospel to the world.
Buddha also told the story of the grieving mother and the mustard seed. When a mother loses her only son, she takes his body to the Buddha to find a cure. The Buddha asks her to bring a handful of mustard seeds from a family that has never lost a child, husband, parent or friend. When the mother is unable to find such a house in her village, she realises that death is common to all, and she cannot be selfish in her grief. 
Mustard seeds have been suggested as a possible source of biodiesel in Australia.