Several different sects emerged after the death of Buddha. The more conservative group came to be known as Theravada Buddhism.
The Theravada belief system is structured on early scriptural writings and the teachings of the Buddha. While it acknowledges the efforts of an individual, there is no way of reaching Nirvana in this lifetime. Only through rigorous effort can an individual reach a level of piety or monk-hood that may allow him to either be reborn as something better in the next life or achieve Nirvana. Theravada Buddhism rejects the notion of praying to Buddha because Buddha has already ascended into Nirvana. The active practice of Theravada followers is done by meditation.
The more liberal and magnanimous group opposing the Theravada belief system is Mahayana Buddhism. Accepting the basic beliefs of Theravada Buddhism, the Mahayana school of thought allows for the salvation of everyone and not just for the extremely pious. Another difference arises with the Mahayana doctrine of the Bodhisattva. The Bodhisattva is an individual who postpones his goal of reaching Nirvana in order to aid his fellow man. Mahayana Buddhism also teaches the doctrine of Buddha “Bodies.” Buddha “Bodies” are essentially projections of a one universal Buddha that is manifested in different forms with the goal of aiding mankind. The active practice of Mahayana followers includes meditation, vows, prayers, and visualizations.
The latest arriving school of thought is Vajrayana Buddhism. The Vajrayana belief system accepts the core teachings of Buddhism and the doctrine of the Buddha “Bodies.” One Vajrayana doctrine that expands on the Bodhisattva of the Mahayana school is the Dalai Lama. The Dalai Lama is an incarnation of Bodhisattva Avalokitesvara, and he is held in the highest honor in the Vajrayana belief system. Furthermore, realization and salvation can be reached by anyone, much like the Mahayana school. The active practice of Vajrayana followers includes all of the Mahayana practices, Tantra, which is mystical-erotic energies, mantras, and different styles of yoga.
Although there has been the formulation of different schools of thought within Buddhism, all sects carry the core teachings of the Four Noble Truths and practices to achieve the coveted state of Nirvana.