This is a quick overview of some aspects of Aristotle's ethics. For links to many excellent internet resources on Aristotelian ethics, see the Aristotle and Virtue Ethics section of Lawrence Hinman's Ethics Updates site.
What is intrinsically good?First, some quotations from Book I of the Nicomachean Ethics. Aristotle does not use the term "intrinsic good," but does talk about:
§2: "some end of the things we do, which we desire for its own sake"
§4: "the highest of all goods achievable by action"
§7: "we call that which is in itself worthy of pursuit more final than that which is worthy of pursuit for the sake of something else, and that which is never desirable for the sake of something else more final than the things that are desirable both in themselves and for the sake of that other thing, and therefore we call final without qualification that which is always desirable in itself and never for the sake of something else."
So, we have the following kinds of goods (again, in a non-Aristotelian vocabulary):
instrumental goods: wealth, flutes
mixed goods (both instrumentally and intrinsically good): honor, pleasure, reason, virtues
intrinsic good (final end): happiness
What is Happiness?
a. The Greek word usually translated as "happiness" is eudaimonia. Given the contemporary prevalence of hedonistic attitudes about happiness, this may be somewhat misleading. The term means something like "human flourishing" or thriving; this is suggested, for instance, in Aristotle's description of happiness as "living well and doing well" (§4).
b. Happiness is objective, not subjective. (Compare: those plants are really happy there. We don't mean that the plants are having pleasant experiences; we mean that they are doing well there, and whether they are doing well is an objective matter, not a matter of how they feel.)
c. So happiness doesnt have much to do with feelings, e.g. of pleasure; rather has to do with whether you have your act together, so to speak. But it has to do with the thriving of the soul rather than the body (§ 13, on virtue).
d. Happiness is an activity rather than a state: "activity of the soul in accordance with virtue," Aristotle says, which introduces our next question.
What is Virtue?
Virtues are states of character conducive to happiness, i.e. to flourishing ("the state of character which makes a man good and which makes him do his own work well" - §II.6)
There are two main categories of virtues: intellectual virtues concern only what Aristotle calls the rational part of the soul, while moral virtues involve both the rational and the appetitive (or desiring) part of the soul: moral virtue involves having the passions under rational control.
In particular, the virtues involve having the right amount of a particular passion, or engaging in a particular kind of action to the right extent. So they have just as much to do with feeling as with doing: feel sympathy and pity where appropriate, e.g.; feel anger when appropriate and not otherwise; and so on
If virtue involves acting or feeling in a certain way to the right extent, what is the right extent? Aristotle says that it is a mean between extremes, but not a mechanically determinable mean: "to feel them at the right times, with reference to the right objects, towards the right people, with the right motive, and in the right way" (§II.6, 427)
The virtues are acquired through habituation, not through instruction (end of §II.4: listening attentively wont make you good): through practice, roughly.
What is Ethical Theory Good for?Well, it wont make you good! It enables you to understand what goodness is, and may perhaps influence the way you raise your children. So its not about finding a decision procedure by means of which to resolve moral problems or dilemmas. (End of §II.1: "it makes no small difference, then, whether we form habits of one kind or of another from our very youth; it makes a very great difference, or rather all the difference. . . .") This is the fundamental difference between Aristotelian ethics and more modern versions; modern moral theory has been largely concerned with determining how to act in particular situations.