Hardy plant of the Brassicas Cabbage family with large dark ruffled leaves and a very pungent flavor.  It's seeds are a spice used in cooking but the young leaves can also be added to a salad to give it a bit more tang.

The active ingredient that gives Black Mustard it's bight is called Isothiocyanates, which is also found in Rocket, Cress and Horseradish.

Once the plant matures it's leaves are too large and pungent to be used in salad.  So, except for the plants I want for seed,  I usually pull them out well before they grow to full size.