Theravada, the most ancient form of Buddhism, is the dominant school in Southeast Asia (Thailand, Myanmar/Burma, Cambodia, and Laos). Its name translates to "Doctrine of the Elders," and it centers around the Pali scriptures, transcribed from the oral tradition taught by the Buddha. By studying these ancient texts, meditating, and following the eightfold path, Theravada Buddhists believe they will achieve Enlightenment. Strong emphasis is also placed on the monastic community and on heeding the advice of the wise.
Mahayana Buddhism developed out of the Theravada tradition roughly 500 years after the Buddha attained Enlightenment. A number of individual schools and traditions have formed under the banner of Mahayana, including Zen Buddhism, Tibetan Buddhism, Pure Land Buddhism, and Tantric Buddhism. Mahayana Buddhism focuses on the idea of compassion and touts bodhisattvas, which are beings that work out of compassion to liberate other sentient beings from their suffering, as central devotional figures.
Vajrayana was last of the three ancient forms to develop, and provides a quicker path to Enlightenment than either the Theravada or Mahayana schools. They believe that the physical has an effect on the spiritual and that the spiritual, in turn, affects the physical. Vajrayana Buddhists encourage rituals, chanting, and tantra techniques, along with a fundamental understanding of Theravada and Mahayana schools, as the way to attain Enlightenment.
Zen Buddhism is said to have originated in China with the teachings of the monk Bodhidharma. Zen Buddhism treats zazen meditation and daily practice as essential for attaining Enlightenment, and deemphasizes the rigorous study of scripture.
Because Buddhism is a system based on practice and individual experience rather than on theology or dogma, the different forms that have emerged differ less in what they believe the Buddha's teachings to be than in how they believe Buddhism should be practiced in daily life.