Mustard as a spice is an important part of Indian cooking.The whole Mustard seeds are used for tempering, to add flavor to the oil. The Mustard paste is very popular in mostly Bengali or east Indian cooking. Mustard Oil is also an important part of Bengali Cooking and traditional Bengali Cooking mostly used Mustard Oil as the cooking medium. Even now when many households in Bengal have grown more accustomed to the other white oils, Mustard Oil is still the favored oil for frying and cooking fish. A recent study shows that Mustard Oil as a cooking medium reduces the chance of heart disease by 70% and it also an excellent source of anti-oxidants (source: Wiki)
There are three kinds of Mustard seeds -- Black, Brown and White/Yellow depending on the Mustard plant.
Mustard paste or oil adds a sharp, spicy taste to food. Both the Brown and Black Mustard seeds are used in Indian cooking, the brown one more popular for making the paste.
Brown Mustard seeds are considered good for digestion and for alleviating stomach discomfort as gas and cramps (source: Maharishi Ayurveda) Mustard Oil is also widely used for massage in Northern India. Even now when my daughter catches a cold, I heat a spoonful of mustard oil with garlic flakes and rub the warm oil on her chest and on the bottom of her feet. My dad says, his grandma used to tell them to rub mustard oil on their big toe before bath as that helps eyes to remain healthy, this piece of information is not proven though and might be a lore Mustard seeds were used medicinally not only in India but also by the Greek and the Romans
Update: From the coments I gather that many of you are unaware of use of mustard paste in Bengali Cooking. We grind mustard to a paste with green chillis and salt and use that extensively in many of our cooking. This cannot be substituted with the Mustard Sauce we get in stores here as it lacks that sharpness. A particular mustard based sauce called "Kasundi" is very popular in Bengal. It is used as a dip and is best when eaten with rice and alu seddho (boiled potatoes) or any sauted greens. Sorshe Bata or Mustard Paste is ubiquitous ingredient in Bengali cuisine. The best Bengali fish curries always have a mustard based sauce. Even vegetarian dishes like Shukto, Charchari (shall blog soon) etc. have mustard paste as the only spice. The black mustard when ground to a paste may be slightly bitter so it is ground with a little poppy seeds (optional) and green chillis and salt. This somehow never happened at home when the Shil Nora was used for grinding. Radhuni is not mustard, it looks more like Ajwain and maybe in the same family, but it is not Ajwain and has a stronger smell & flavor. Its hard to get Radhuni outside Bengal and I don't have any.
So add that little Mustard to your food in whatever form you like best and enjoy.
Trivia: Aristocrat Nikolaus Ludwig von Zinzendorf founded the Order of the Mustard Seed in Germany in 1715 inspired by the Parable of Mustard Seed told by Jesus
Predominantly a Bong, who loves being a Mom and loves to cook among other things for the li'l one and the big ones.She loves to write too and you will find her food spiced up with stories. Mainly a collection of Bengali Recipes with other kinds thrown in, in good measure. A Snapshot of Bengali Cuisine