Mustard powder is really just ground yellow mustard seeds, though some commercial brands add turmeric for a darker colour. We categorize it separately since it tends to have different uses than whole mustard seeds and many people think of them separately. While yellow mustard seeds are used in pickling and preserving, mustard powder is used in making barbeque sauces, salad dressings and, of course, mustard.
Mustard powder is quite pungent smelling and has some heat to it, but it goes stale relatively quickly, so don’t keep it in the cupboard too long. Also, when cooking with mustard, always add it toward the end of the process, since too much heat dissipates the flavour.