Primarily, Theravada Buddhism focuses more on the individual while Mahayana Buddhism focuses more on society as a whole. They also concentrate on different values: Theravada on wisdom and Mahayana on compassion.
Theravada Buddhism was the first type of Buddhism to develop, so it is closest to the original teachings of the Buddha. It is more focused on individuality than other forms of Buddhism, meaning that is up to the individual to reach enlightenment. According to the University of Wyoming's Glossary on Buddhism, the individual uses meditation to reach enlightenment while still alive. Nirvana is the goal after death.
While Mahayana Buddhism still focuses on reaching enlightenment, an individual's purpose isn't to just pass into the perfect peace of nirvana; rather, a Mahayana Buddhist should come back to life after reaching enlightenment to help others. More people follow Mahayana Buddhism than any other branch.