The Theravada Buddhists believe that they practice the original form of Buddhism as it was handed down to them by Buddha. Theravada Buddhism dominates the culture of Sri Lanka, but is also very prominent in Thailand and Burma.
While Siddhartha Gautama, the Buddha, spent several decades teaching, none of his teachings were written down until several hundred years later. In the third century, Asoka, the great Mauryan emperor, converted to Buddhism and began to sponsor several monasteries throughout the country. He even sent missionaries out to various countries both east and west. During his reign, the teachings of Buddha spread all across India and Sri Lanka.
Disturbed by the prolific growth of Buddhist heresies, a council of Buddhist monks was convened at the Mauryan capital of Patna during the third century BC to purify the doctrine. What arose from that council, more or less, were the definitive teachings of Theravada Buddhism; from this point onwards, Theravada Buddhism undergoes little if any change.
When the teachings of Buddha were finally written into a canon, they were written not in Sanskrit, but in a language derived from Sanskrit, called Pali. This language was spoken in the western regions of the Indian peninsula, but from Sri Lanka (which is off the eastern coast of India) to Burma, the Pali scriptures would become the definitive canon. We can' determine precisely when they were written down, but tradition records that the canon was first written down somewhere between 89 and 77 BC, that is, over four hundred years after the death of Buddha.
This canon is called the Tripitaka, or "Three Baskets," for it is divided into three parts, the Vinaya , or "Conduct," the Sutta , or "Discourses," and the Abhidhamma , or "Supplementary Doctrines." The second part, the "Discourses," are the most important in Buddhism. These are discourses by the Buddha and contain the whole of Buddhist philosophy and morality.
The basic doctrines of Theravada Buddhism correspond fairly exactly with the teachings of Buddha. Theravada Buddhism is based on the Four Noble Truths and the idea that all of physical reality is a chain of causation; this includes the cycle of birth and rebirth. Through the practice of the Eightfold Noble Path and the Four Cardinal Virtues, an individual can eventually attain Nirvana . Theravada Buddhism, however, focussed primarily on meditation and concentration, the eighth of the Eightfold Noble Path; as a result, it emphasized a monastic life removed from the hustle and bustle of society and required an extreme expenditure of time in meditating. This left little room for the bulk of humanity to join in; Theravada Buddhism was, by and large, an esoteric religion. A new schism then erupted within the ranks of Buddhism, one that would attempt to reformulate the teachings of Buddha to accomodate a greater number of people: the "Greater Vehicle," or Mahayana Buddhism.