There are two major types of mustard seed: white (or yellow) and brown (or Asian). A third species, the black mustard seed, has been replaced for most purposes by the brown species because the latter can be grown and harvested more economically. 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.
From The Food Lover's Companion, Fourth edition by Sharon Tyler Herbst and Ron Herbst. Copyright © 2007, 2001, 1995, 1990 by Barron's Educational Series, Inc.