The Brassica genus includes broccoli, cabbage, cauliflower, turnips and radishes. The mustard family also includes plants grown for their leaves, like arugula, a number of Oriental greens, as well as mustard greens. Three related species of mustard are grown for their seeds:
White Mustard: (Brassica alba or Brassica hirta) is a round hard seed, beige or straw coloured. Its light outer skin is removed before sale. With its milder flavour and good preservative qualities, this is the one that is most commonly used in ballpark mustard and in pickling.
Black Mustard: (Brassica nigra) is a round hard seed, varying in colour from dark brown to black, smaller and much more pungent than the white
Brown Mustard: (Brassica juncea) is similar in size to the black variety and vary in colour from light to dark brown. It is more pungent than the white, less than the black.
Bouquet: The seed itself has no aroma.
Flavour: Sharp and fiery.

OVERVIEW

It was the condiment, not the plant, that was originally called mustard. The condiment got its name because it was made by grinding the seeds of what was once called the senvy plant into a paste and mixing it with must (an unfermented wine). Mustard is one of the oldest spices and one of the most widely used. The Chinese were using mustard thousands of years ago and the ancient Greeks considered it an everyday spice. The first medical mention of it is in the Hippocratic writings, where it was used for general muscular relief. The Romans used it as a condiment and pickling spice. King Louis XI would travel with his own royal mustard pot, in case his hosts didn't serve it. Today, world consumption of mustard tops 400 million pounds.

Tadka or Tempering is a cooking method in which cooking oil is heated till very hot and whole spices are added to it and fried. This oil and spice mix is then added as a final touch or garnish to the dish. In Indian cooking, Rai/ Sarson is often part of the Tadka in a dish. In comparison, Rai/ Sarson is used more in South Indian than in North Indian cooking. In the South, it is typically combined with green chillies and curry leaves in a tadka. It is sometimes used in paste form, in Eastern Indian dishes like Deemer Patudi or fish curries.