Plato and Aristotle are similar in that they both contemplated man's existence in the world and the significance of that existence. They both tried to understand what it means to be aware of one's existence and how that existence is related to that of others.
Plato was Socrates' student, and Aristotle studied as a student of Plato. Although he disagreed with his teacher about some key points, much of Aristotle's work is considered an evolution of the ideas first put forward by Plato. Plato's work, however, was more individually focused and concerned with the soul.
Aristotle was more of a political thinker who tried to put his ideas into a social context. To him, it was impossible to consider one without considering the other. Much of his scientific thinking was based on this same principle. To Plato, being was being. To Aristotle, observing was being.
Many of Aristotle's ideas are a marriage between those of Socrates and Plato. It is said that Plato was the first political philosopher and Aristotle was the first political scientist. Both men founded schools. Plato's school was The Academy, at which Aristotle studied. Aristotle's school was the Lyceum, which was partially financed by Alexander the Great.