Curry powder is a very mild spice mix originally concocted in the 19th century to help British people make things taste like Indian food with minimal effort and minimal risk of over-spicing. The two main ingredients of curry powder are turmeric powder (which gives it its signature yellow color) and cumin powder. These two ingredients will always be present in large amounts. Other common ingredients include ground coriander seeds, ground black pepper, garlic powder, ginger powder, ground cardamom, ground black mustard seeds, cayenne powder, and ground cinnamon. These are all much stronger spices and are found in very small amounts, if at all.
Real Indian cooking does not use curry powder, although it uses the components thereof. Curry powder is a very mild formulation and is much weaker than the much more potent garam masala, a real Indian spice mix actually used in Indian cooking. The two are not interchangeable. You would normally add much more curry powder to a dish compared to just a small amount of garam masala.