One of the biggest differences in these two traditions is the goal of training. Theravada is known for its Arhat-ideal. This means someone who has attained the goal of religious life. This goal of the religious life is to get to enlightenment or Nirvana. This enlightenment is when a buddhist discovers the truth about life. Buddhists generally achieve this enlightenment by meditation or putting into practice the eight-fold path that they follow. Theravada is written in Pali canon, the sacred text of this tradition.
On the other hand, Mahayana teaches whats called a bodhisvatta-ideal. This is when a person can achieve enlightenment but chooses not to in order to save suffering beings out of compassion. Traditionally bodhisavatta is anyone who is compelled by compassion and has reached a mindset of enlightenment and has a wish to become like buddha for all beings. They wish to set free others from this cycle of death, rebirth, and suffering. Mahayana is written in Sanscrit.
Mahayana and Theravada are very different teachings of Buddhism. Some even say that Mahayana is not real or credible because it differs from the teachings of the historical buddha. But these are both very popular traditions and are widely practiced. They do have similarities in that they all want to reach enlightenment somehow. Whether they get there and achieve their goal or if they can get there but choose to help others get there out of compassion instead.