Roy F. Baumeister and a team of social psychologists recently studied the distinction:
If happiness is about getting what you want, it appears that meaningfulness is about doing things that express yourself. Even just caring about issues of personal identity and self-definition was associated with more meaning, though it was irrelevant, if not outright detrimental, to happiness. This might seem almost paradoxical: happiness is selfish, in the sense that it is about getting what you want and having other people do things that benefit you, and yet the self is more tied to meaning than happiness. Expressing yourself, defining yourself, building a good reputation and other self-oriented activities are more about meaning than happiness.
The Dish 