Short answers, pulled from the story.
Theravada is Buddhism's oldest existing school, and its followers have preserved their version of the Buddha's teaching in the Pali Canon for over two millennia. The name means "Doctrine of the Elders," combining thera, meaning elder or senior monk, with vada, meaning doctrine or teaching. As of 2010 it was the second largest branch of Buddhism, with 36 percent of Buddhists belonging to it compared to 53 percent for Mahayana.
Theravada is the official religion of Sri Lanka, Myanmar, and Cambodia, and the dominant Buddhist variant in Laos and Thailand. It is also practiced by minorities in India, Bangladesh, China, Nepal, Vietnam, Indonesia, and Malaysia, as well as by diaspora communities and converts around the world.
The Pali Canon, or Tipitaka, is the most complete Buddhist canon surviving in a classical Indian language and contains the main scriptures of Theravada. It was committed to writing in the first century BCE and has three parts: the Vinaya Pitaka, the Sutta Pitaka, and the Abhidhamma Pitaka. According to Alexander Wynne, surviving inscription fragments agree almost exactly with extant manuscripts, meaning it was transmitted accurately for well over 1,500 years.
Theravada rejects the authenticity of the Mahayana sutras, which appeared from around the first century BCE onward, and generally does not recognize many Buddhas and bodhisattvas such as Amitabha and Vairocana because they are not in its scriptures. It also holds that the Buddha resided in the human realm, rejecting the Mahayana view that his body was an incorporeal manifestation and his birth and death an illusion.
Buddhaghosa was an influential scholar of the fourth and fifth centuries CE, known for his Pali commentaries based on older Sri Lankan commentaries of the Mahavihara tradition. He wrote the Visuddhimagga, a compendium of Theravada doctrine that became the orthodox account of the path to liberation and systematized meditation into 40 forms called kammatthanas.
Theravada became dominant in the Southeast Asian kingdoms of Sri Ksetra and Dvaravati from about the 5th century CE, and from around the 11th century Sinhalese monks and Southeast Asian elites led a widespread conversion of the mainland. The Burmese king Anawrahta, who reigned from 1044 to 1077, and the Thai king Ram Khamhaeng, active in the late 13th century, helped make it the predominant religion of Burma and Thailand.