Dharma Data: Buddhist Traditions
As with other religions, Buddhism is not a single system of belief and thought but a collection of numerous schools differing in particulars while being united by certain common features. The history of the development of the different traditions within Buddhism is obscure. The earliest divisions between Buddhists were due to disagreements about Vinaya, although this probably did not lead to them breaking into different schools or sects, something that occurred only later (2nd century BC) when doctrinal disputes began to emerge. The first schools together made up the earliest Buddhist tradition, the Savakayana.
N. Dutt, Buddhist Sects in India. Calcutta, 1970.