After Buddha Sakyamuni entered parinirvana, his disciples proceeded to explain, in accordance with their own understanding and realization, the teachings which the Buddha expounded. The Dharma teachings which are the actual words of the Buddha are called sutras; the commentaries of the disciples are called sastras. In the Chinese edition of the Tripitaka, collections of both the sutras and the sastras can be found.
There is an extensive compilation of sutras and sastras in Buddhist literature. Many of the Tibetan monasteries and Buddhist institutes of advanced studies have included five types of sastras in their required curriculum. They are called the five major categories of sastras.
~ Depicted from ARE U READY FOR HAPPINESS : The Significance of Buddhist Philosophy Today