Sruti, Smrita and Nyaya
"The Vedic literatures are divided into two parts: the srutis and the smrtis. The srutis are the four Vedas: Rg, Sama, Atharva and Yajur, and the Upanisads, and the smrtis are the Puranas like the Mahabharata, which includes Bhagavad-gita. The conclusion of all these is that one should know Sri Krsna as the Supreme Personality of Godhead."
Krsna book, Chapter 87
Vedic injunctions are known as sruti, and smrti are the presentations made by great sages of the principles contained within sruti. Nyaya is the logical process of arguing these principles in order to gain understanding of them.
The Upanisads and Veda Samhitas are known as sruti-prasthana. The Puranas along with the epic Mahabharata, which includes the Bhagavad-gita, are known as smrti-prasthana. The Vedanta-sutras, which present Vedic knowledge on the basis of logic and arguments is known as nyaya-prasthana. These three sources of Vedic knowledge are called prasthana-traya.
Just as all spiritual matters must be tested according to guru, sadhu and sastra, all scientific knowledge of transcendence must be supported by sruti, smrti and nyaya. Smrti and nyaya always confirm that which is said in the sruti.
Scripture follows the principles of the six limbs of Vedic knowledge known as Vedangas. The six aspects are:
Siksa - phonetic science
Vyakarana - grammar
Nirukti - context (conclusive meaning)
Candas - meter
Jyotis - time science (astronomy & astrology)
Kalpa - rituals
Sutras, which are short but potent phrases explaining the meanings of each aspect of Vedangas, are composed by those advanced in Vedic knowledge. There are four categories of Kalpa-sutras, which provide instruction regarding collective sacrifices (srouta), family rituals (grhya), occupational duties (dharma), and the building of sacrificial fireplaces, altars, etc. (sulba).
Different levels of instruction are provided for those of varying levels of consciousness, who wish to offer worship to different controllers or demigods in order to gain various gratifications.
Agamas, or emanated scriptures, offer instruction in five additional categories, including energy/sakti (sakta agamas), visible source/sun (surya agamas), controller (ganapatya agamas), destroyer/Siva (saiva agamas), and the ultimate source, Visnu (vaikhanasa gamas). For those unable to meet the standards of purification required of other Vedic processes, Lord Siva gave the Tantric sastra. The Tantra is categorized as the right and left aspects. The right aspect provides regulations for purification for those engaged in gross meat eating, intoxication and illicit sex, which the left aspect provides instruction for low class activities like black magic.
While all these instructions are provided in the Vedas, they are not put forth as the best path for self-realization. Rather, the Vedas instruct that one must surrender in devotion to complete loving service of the Supreme Personality of Godhead and his eternal consort, Sri Sri Radha-Krsna. No other path is complete.
Bhaktivedanta Book Trust. HDG A.C. Bhaktivedanta Swami Srila Prabhupada.