For Aristotle, virtues, or moral excellences, are things that we do. In order to be virtuous one must act virtuously. The most important virtue of all is what Aristotle calls phronesis. This is a practical virtue which determines when it is appropriate to act in a certain way.
So, for instance, in some cases it will be appropriate to display the virtue of courage. In others, such courage would appear reckless and foolish. It all depends on the precise circumstances. And it is the virtue of phronesis, a kind of practical wisdom, that determines which of the other virtues are appropriate for the situation.
It is only by acting virtuously that we can be considered good. But this must happen over the course of someone's life. It's not enough for Aristotle that we do the right thing every now and then. We must act in a virtuous manner consistently throughout the whole of our lives.
One of the main criticisms that has often been leveled against Aristotle's virtue ethics is that it lacks the universality of a true moral philosophy. In other words, it doesn't apply equally to everyone in all circumstances. The specific virtues that Aristotle endorses, so the argument runs, just so happen to correspond to the kind of values prized by Greek aristocrats such as himself. As such, they have no real applicability outside this charmed circle of elite Greek men.