This is from Wikipedia.
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 .
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.