Mustard seeds are the small round seeds of various mustard plants. The seeds are usually about 1 to 2 millimetres (0.039 to 0.079 in) in diameter.Yellow mustard seeds are an excellent source of selenium and a very good source of omega-3, fatty acids and manganese. Yellow mustard seed is an extremely nutritious and well known herb. It has a sharp aromatic flavour. It prevents cancer. It serves as a wonderful beauty aid as well. It is commonly used in pickles and south Indian dishes.