The 5 Branches of Philosophy
Life As the Moral Standard
Reason Is Man's Means of Survival
Harmony Of Interests
We are what we repeatedly do. Excellence, then, is not an act, but a habit.
A virtue is a moral habit which generally results in the gaining or maintaining of your values. Your values are based on your moral standard which should be your own life. Virtues are pre-thought out methods for achieving your values. This means that with rational virtues, acting virtuous leads to a happy and successful life.
This is very different from the traditional mystical view that there is some "good" out there which is opposed to your natural tendencies and you constantly have to choose between what you want and what is "good". There is no choice to be made between some "mystical good" and your own life, morality is not a limit on action. What is "good" is actually that which is in your rational self-interest -- there is no conflict.
It is important to keep in mind that virtues are not absolutes. Or, put another way, they are contextually absolute. They are not to be followed blindly and dogmatically. Virtues only apply within the context in which they were formulated. To understand the context and when a virtue applies is why you must understand the "why" behind the "what" of each principle. When it is not clear whether a virtue applies or how to apply it, you must fall back onto your ultimate standard of value, your life, to guide your actions.