Curry Powder is more than just chilli and turmeric. There are a variety of spices combined together in each mix, which reflect regional tastes. Some of these, such as the Balti have developed as a result of curry becoming popularized within the UK.
Garam Masala is considered a sort of "staple spice" of India. It is a mix of typical curry spices including coriander, cumin, cloves, cumin, pepper, cardamom, chillies and anise. It is usually sold quite mild, and you can add chilli to taste.