According to Aristotle, what are Virtue and Eudaimonia and why are they relevant for the study of ethics?

According to Aristotle, what are Virtue and Eudaimonia and why are they relevant for the study of ethics? What are the possible pros and cons of having a theory or morality centered in the agent (person performing the actions) and not centered on actions themselves? Remember to use sufficient supportive evidence from to explain why you agree or disagree.