Wasabi, or Wasabia japonica, is part of the family Brassicaea which includes horseradish and mustard. It is primarily cultivated for its rhizome, which has a fiery, mustard-like flavor and pungent fragrance.
Wasabi is difficult to grow, which makes it expensive to buy. Wasabi is sold as a rhizome or as a dried powder. Fresh wasabi is served with a grater and served to the table right away, because it loses flavor very quickly once it's made into a paste. Fresh Wasabi root will keep, wrapped and refrigerated, for up to two weeks.