The first division
The first lasting split was between the Sthaviras, 'the Elders', and the Mahāsāṅghikas, 'the Great Community'. Both then divided again. Buddhist writers later spoke of eighteen schools, but their lists do not agree, and more than eighteen names are known.
What the schools argued about
Some debates were about discipline. Others were about the finer points of teaching, worked out in a new kind of literature, the Abhidharma, which set out to analyse experience into its basic elements. The Sarvāstivāda school held that these elements exist in past, present and future. The Pudgalavāda held that there is a 'person' who is neither the same as the aggregates of body and mind nor different from them. Others argued about the nature of an arahant, or whether some states of awakening could be lost.
Schools were defined above all by their ordination lineage and Vinaya. Monks who held different doctrinal views could live in the same monastery.
Which survive
Only one of the early schools survives as a living tradition with its own name: the Theravāda, 'the Way of the Elders', descended from the Sthavira line in Sri Lanka. Two others live on through their monastic codes: the Dharmaguptaka Vinaya is still used for ordination in China, Korea, Vietnam and Taiwan, and the Mūlasarvāstivāda Vinaya in Tibet and Mongolia.
Why it matters
Buddhism was never historically one completely uniform institution.
History and tradition
What historians can say
The history of the schools is reconstructed from their own later accounts, which often disagree, from Chinese translations of their texts and from inscriptions naming them at Indian sites. The broad picture is clear; many details, including exact dates, are not.