White mustard seeds are much larger than the brown variety but a lot less pungent. They're the main ingredient in American-style mustards. White and brown seeds are blended to make english mustard. Brown mustard seeds are used for pickling and as a seasoning, and are the main ingredient in European and Chinese mustards. Mustard seeds are sold whole, ground into powder or processed further into prepared mustard.
Powdered mustard is simply finely ground mustard seed.
Mustard seeds can be stored for up to a year in a dry, dark place and powdered mustard for about 6 months. Whole seeds are used for pickling, flavoring cooked meats and vegetables and as a source for freshly ground mustard. Powdered mustards and freshly ground seeds are used in sauces, as a seasoning in main dishes and as an ingredient in salad dressings.