Chili powder (also spelled chile powder) is a generic name for any powdered spice mix composed chiefly of chili peppers, most commonly either red peppers or cayenne peppers, which are both of the species Capsicum annuum. The spice mix may simply be pure powdered chilis, or it may have other additives, especially cumin, oregano, and garlic.  Some mixes may even include black pepper, cinnamon, cloves, coriander, mace, nutmeg, or turmeric. As a result of the various different potential additives, the spiciness of any given chili powder is incredibly variable. As a rule, the purer the chili powder is, the more spicy it is.
Use of powdered chili peppers can be traced, at its earliest, to Indian cuisine, where such powders were used in curry dishes, but today they are especially popular in American cuisine, where they are the primary flavor ingredient in chili con carne. The first commercial blends of chili powder in the U.S. were created in the 1890s by D.C. Pendery and William Gebhardt for precisely this dish.