There are 3 main varieties of mustard: white, brown and black. I tend to use black mustard seeds and they are generally used in tempering of spices or (baghar of spices).
The seeds are thrown into a splash of hot oil and allowed to pop and crackle and it is this that gives a distinctive flavour and smell to certain dishes.
Stand back whilst these seeds pop, they taste great but you wouldn’t want one in your face.