Aristotle's Ethics is focused on having excellent and well-chosen habits. His ethics is a study of how people should live in order to obtain happiness. One of his beliefs was that everything humans do is aimed at some good. Other points of his include moral virtue, justice, and the highest good.
Like many great thinkers in the western tradition, Aristotle valued reason and believed that a good life meant, in part, to live in accordance with reason. It was reason that separated humans from animals, and therefore to be a good human being meant, to a great degree, being reasonable and rational.
Aristotle wrote three treatises on this subject so it is a little hard to boil it down to a post of this length...
The basic idea is that the highest human good is "happiness." Happiness comes about when we fulfill our basic nature as human beings. What makes us human is our ability to use reason. Therefore, we fulfill our nature by using reason. What we are supposed to do, then, is to life a life that allows us use our reason. In general, this means that we should do everything in moderation -- we should not do anything too little or too much.
The Goden Rule states that there is a mean between two opposing proportionals.
The opposing extremes are of two kinds: The deficient and the excessive. And what the good man always seek is the mean(the intermediate) between these two opposing extremes.
Take for example the virtue courage. Hence the good man always seeks the mean, he does not want to be either deficient nor excessive in his courageousness, but only courageous.
So what is deficient? Well, that must be cowardice, hence the good man do not seek lack of courage. Therefore he cannot be deficient in his virtue, since that is not true moral excellence. So what is excessive? That must be rashness, hence the good man do not want to act hastily in his actions, since then he will ignore the concept of reason which is true excellence.
So what we have left is the mean, and that must courage. It is neither to excessive in virtue nor deficient, therefore courage must be the best and the virtue which the good man seeks.
The same can be done with Ready Wit which is the mean, Boorishness the deficient and buffoonery which is the excessive.