All of these words mean the same thing: dry mustard, ground mustard, mustard flour, ground mustard seed, and dry mustard powder. They mean the ground seeds of any of many types of mustard plants.
Mustard powder has a hot taste that can be mild or strong, and it is used in many meat, fish, chicken, and veggie dishes. Different kinds of mustard plants make different kinds of mustard.
Mustard seeds that are white or yellow are the biggest and have the mildest taste. These are used in the typical yellow mustards you get at a ballpark.
Brown mustard seeds, also called Asian mustard seeds, are smaller and have a sharper taste. They are used to make pickles and European and Chinese mustards.
The black mustard seeds are the smallest and most pungent of the three kinds. You don’t see them very often because they are hard to grow and the brown ones have mostly taken their place on the market.
You can also mix mustard seeds together. In English mustards, for example, white and brown mustard seeds are often used together. Some recipes call for mustard greens, which are the leaves of the mustard plant.
In a cool, dry place, mustard seeds can be kept for up to a year. Mustard that has been ground or turned into a powder can be kept for up to six months.