Mustards belong either to the Brassica or Sinapsis genera. Mustards are upright cool-season annuals that grow 3-5 feets tall. They have narrow leaves, yellow flowers, a strong taproot, and fibrous and lateral roots.
Brassicas & mustards overview: Managing Cover Crops Profitably (SARE Handbook Series Book 9) by Andy Clark (2007, 3rd ed.)