Classical Hinduism is known as Sanatana Dharma which means Eternal and Ancient Law, or the Law that was given to mankind by the devas (the incarnated gods) in the primordial times. The foundation of Sanatana Dharma is forged from Śruti (that which is heard), and its structure is forged from Smṛti (that which is remembered).
The Śruti consists of the four Vedas (the Rigveda, the Yajurveda, the Samaveda and the Atharvaveda) which the ancient sages received directly from the devas. The Vedic teachings were not written down for several millennia and were preserved through an oral tradition. The teacher would sing the Vedic hymns to his pupils who, when they became teachers, would in turn sing the hymns to their own pupils—thus the Vedic teachings were passed in a pristine form from generation to generation. The Vedas have four parts: the Samhitas (mantras and benedictions); the Aranyakas (explanation of the rituals, ceremonies, and sacrifices); the Brahmanas (commentaries on rituals, ceremonies, and sacrifices); the Upanishads (discussion of meditation, philosophy, and spirituality).
The Smṛti (that which is remembered) consists of the sacred texts which are composed by the ancient sages: this tradition consists of Manusmriti, Yājñavalkyasmriti, Sankha Likhita Smriti, Parashara Smriti, the Dharmasūtras and Dharmaśāstras, the eighteen Mahapuranas (Major Puranas) and eighteen Upapuranas (Minor Puranas), the ancient epics (the Mahabharata and the Ramayana), the Arthasaśāstras, and several other texts. Yājñavalkya, the sage who flourished between seventh and eighth centuries B.C., gives the name of twenty Smṛti texts, several of which are no longer extant.
In some classifications, a third category is added to the structure of Sanatana Dharma—it is called Itihasa (history). The two epics, the Mahabharata and the Ramayana, are part of the Itihasa category. There is also the category of Tantra, which means the science of doing practical things. Much of ancient Tantra is no longer extant. The ancient sages felt that people in ancient times were not ready for such knowledge and they stopped teaching the Tantric arts.