Many people think of mustard first as a plant or a condiment, rather than a spice; but it's actually all three, and more. Some of the plants we eat as mustard greens or other types of brassica also yield the mustard seed, which is then made into the condiment (of which there are many variations) or mustard oil, and can even be used to treat certain types of illness.
Mustard seeds tend to be classified by colour - such as white, yellow, brown or black. Brown and black seeds are much more pungent than the lighter coloured ones because they contain sinigrin, the compound that's also found in horseradish. Sinigrin is released only when the seeds are crushed, which can be by chewing or grinding them.
The tiny seeds are used in many cuisines, and are often blended with other spices, including other types of mustard.
When whole mustard seeds are used in dishes, they're usually heated - by themselves or with other ingredients - in oil, until they start to pop. The flavour of whole seeds is much milder than when they're finely ground.
If you want to make your own mustard, the seeds need to be ground. They're usually soaked in liquid, which softens the tough husk. The seeds are ground with other ingredients - often something acidic and something sweet (to balance the acidity); spices; and aromatics. It's best to make the mustard well in advance of cooking with it so the flavours have time to blend and soften, otherwise it will be too fiery.