Buddhism began in India from the teachings of Siddhartha Gautama. This original school of Buddhism, which adheres to the teachings of the first awakened Buddha, is known as Theravada Buddhism. Over time, Buddhism spread into Southeast Asia, where it began to take a new form. Deities like Sitatapatra began to appear as Buddhism moved through Myanmar, Thailand, Laos, and Vietnam. By the time Buddhism moved north into China, it had grown far beyond just the teachings of Gautama Buddha. Instead of the individual pursuit of enlightenment, it was believed reaching Nirvana was a collective effort. This is called Mahayana Buddhism, and is the predominant form of Buddhism in the world today. When Buddhism traveled to China over the Himalayas, it got stuck up there and became Vajrayana Buddhism, which is the smallest of the three schools. The final destination of Buddhism’s historical journey was Japan. There, Chon Buddhism from China developed into Zen Buddhism. Pure Land Buddhism also was developed in Japan.
Theravada Buddhism is characterized by a conservative canon of texts and doctrine of an individual path to englightentment. Mahayana is sanskrit for “great vehicle” which refers to the idea that everyone must reach enlightenment together, not separately. Vajrayana Buddhism is characterized by their tantric practices like mandalas, mantras, dharanis, and mudras. It has intense involved practices and rituals like Theravada, but a more religious tactility like Mahayana.
These are called the three schools of Buddhism because all other schools associate with one of these three. For example, Zen Buddhism is a school of Mahayana Buddhism.