Although there are many differences between these two, the most important is that Mahayana is a less rigorous version of Buddhism while Theravada is a more purist approach to the faith.
Mahayana Buddhism is said to have arisen in an effort to compete with salvationist faiths for believers. It was a less stringent interpretation of the faith. According to Mahayana ideas, a person could attain Nirvana more easily than was possible in Theravada. Mahayana held that a person could attain Nirvana essentially by performing religious rites instead of by extremely difficult attention to their personal behavior and attitudes. In addition, Mahayana adherents believed that many boddhisatvas could intercede for them and help them attain Nirvana where as Theravada adherents believed only in the Buddha himself. In these ways, Mahayana can be seen as more of a religion while Theravada can be seen as more of a philosophy.