Chili powder has more than one meaning. In many US recipes it is a seasoning blend containing ground dried chilies with ingredients such as cumin, garlic and oregano; it may also contain salt. In other regions, particularly where spelled chilli powder, it commonly means pure ground dried chilies.
Read the ingredient list. A mild blend and a hot single-chile powder can differ dramatically in heat and flavor, so they are not dependable spoon-for-spoon substitutes. Cayenne, paprika and chili flakes are related products with different uses.
Reference: Further reading.