While you're waiting for Cathe's response, I thought I'd throw in my two cents. I have been practicing Muay Thai (Thai boxing) for over a year. In Muay Thai, we use shadowboxing to practice good form and get cardio exercise. There are some techniques that are harder to practice on a stationary heavy bag, and with shadowboxing you can move around a lot more and practice your footwork. We use heavy bags to practice using techniques with power, punching and kicking as hard (and accurately) as we can. They are more for strength training. However, the line isn't that distinct - if you work fast on the heavy bag you can still get amazing cardio, and if you wear heavy gloves during shadowboxing you can build up your strength.