Full Idea
Principles concern not just types of actions, but one's life as a whole, grasping truths about the nature of justice, and the like; they explain rules, giving the 'why' and not just the 'what'.
Gist of Idea
Principles cover life as a whole, where rules just cover actions
Source
Julia Annas (The Morality of Happiness [1993], 2.4)
Book Reference
Annas,Julia: 'The Morality of Happiness' [OUP 1995], p.104