The curry powder sold in supermarkets is similar to garam masala that comes from India and contains a mixture of ground spices. Traditional Indian dishes do not contain curry powder as Westerners know it; cooks use a variety of blends and ratios of ingredients that differ from one region and one family to the next. In addition, the recipe changes according to the type of dish.
Indian spice merchants invented curry powder for Brits returning to England from India in the 1780s, when the country was a British colony. Some returning personnel had become accustomed to Indian food and they adapted the pre-mixed powders to suit their own tastes.
Curry powder typically contains ground coriander, ground cumin, ground pepper, turmeric and ground ginger. Garam masala is a sweeter mix that contains cardamom, coriander, cumin, black pepper, cloves, cinnamon and nutmeg.