THE 3 BUDDHIST TRADITIONS

WHAT ARE AND WHY 3 BUDDHIST TRADITIONS:

Somebody once asked how one could fit together various traditions that represented the Buddha's teaching. One can think of Buddha's Dharma as a wonderful seed planted in the earth, out of which has blossomed a tree with deep roots, great branches, leaves, flowers, and fruits.
Sometimes a person might point to the roots and say that it is just here that we can find the real Dharma, while someone else might say, "Oh no it is in the flowers," and still another will say that it is to be found in the fruit. But, of course, these different parts cannot really be separated; the roots sustain the tree in their way, and the fruit depends on the roots and leaves and branches as well.
In order to clarify the variations between the many different schools and traditions of Buddhism, the schools are often divided into the three Yanas (Skt.), meaning 'Vehicles' or 'Paths'. These three are; the Hinayana, Mahayana and Tantrayana. Within the various vehicles, much variation can still exist, which is further explained in the pages that deal with the traditions, like Zen and Tibetan Buddhism.
A major reason for this development of different schools within Buddhism may be that the Buddha taught for decades. Given the vast amount of teachings it is not easy to unanimously decide what the exact interpretation of all teachings should be, or even how to summarise hem logically.
Depending on who the Buddha would be teaching to, the explanation would be quite different and sometimes seemingly contradictory. This can be understood as skillful means; a satisfying explanation to a learned philosopher is probably too complex for an uneducated person. On top of this, the Buddha clearly stated that he did not just intend to teach a doctrine, but intended to show the path that people can follow for their own development. This intention ultimately leads to the point where every individual has to decide which practices to follow and how to interpret the teachings, rather than adhering to a fixed doctrine.
Source: http://buddhism.kalachakranet.org/vehicles.html