Aristotle, who was a Greek philosopher from the 4th century BCE, had a very different ethical theory from that of Immanuel Kant, an 18th century German philosopher.
The first important difference is that Aristotle's ethical theory is agent-centered. He was concerned with describing what it was to be a virtuous or good human being, whereas Kant's ethical theory is action-centered. Kant was concerned with describing what good or virtuous actions were. For Aristotle, the right action is that which is done by a good person. For Kant, a good person is one who performs the right action.
Aristotle, in the Nicomachean Ethics, claims that the best kind of life consists in a state of eudaimonia: objective happiness and flourishing. This happiness consists in activity in accordance with reason. Thus, the highest kind of happiness involves a life of theoretical contemplation by an individual with intellectual virtues who has been habituated to moral virtues.
Kant, in his Critique of Practical Reason and other works, describes an ethical view that is often called "deontological." This means that the notion of duty is central to his ethics; he believes that we have perfect and imperfect duties both to ourselves and toward others and that living an ethical life requires us to fulfill these duties. These duties are derived from the moral law that is within each of us.
While the above descriptions highlight the central difference between the two, it is also worth pointing out that there are some notable similarities as well. The most fundamental of these is their shared belief in an objective ethical theory: neither Kant nor Aristotle are ethical relativists and both believe that there is an objective ethical standard. Further, this standard is a universal system since it pertains to all of humanity, irrespective of any differences.