Plato and Aristotle are both great philosophers in their own regard. Both agree that the world has a purpose, and that it's not just an accident. Both also hate materialists since in their (materialists') interpretation of the world, value, choice, and freedom are not plausible outcomes, and so morality and rationality do not make sense. And both ask the same question, what does it take to be a good, moral person? Yet, even though Aristotle was a student of Plato, each philosopher develops his own view on things and a specific way of solving a particular problem.
For example, Plato and Aristotle have quite different views regarding life. Plato is dissatisfied with sense and desire, which are nothing to him except a shadow of reality - his aim of philosophy is to die away from these things. The real, on the other hand, is unchanging, static, eternal. He aims to grasp the Forms and to contemplate the True, the Good, and the Beautiful, and to remain caught up in contemplation of these things.
Aristotle has no such discontents, however. He likes life in this world, even though it is not perfect. He does develop his own view of the divine and how it is related to the world, but sees no reason why one would be driven to flee from life in the world. The various animals he studies are real things; philosophy to him is not to run away from them, but a way of comprehending them.
Plato is committed to the idea that reality is ultimately rational. His Forms are definite realities made up and bonded together in perfectly rational ways, and together they make a perfectly systematic whole. For him, mathematics seems to embody the ideal of knowledge and reason is the only way to discover truth.