The Sruti and the Smriti
The Sruti and the Smriti are the two authoritative sources of Hinduism. Sruti literally means what is heard, and Smriti means what is remembered. Sruti is revelation and Smriti is tradition. Four vedas and Upanishad are Srutis. Sruti is direct experience. Great Rishis heard the eternal truths of religion and left a record of them for the benefit of posterity. These records constitute the Vedas. Hence, Sruti is primary authority.
Smriti is a recollection of that experience. Hence, it is secondary authority. The Smritis or Dharma Sastras also are books written by sages, but they are not the final authority. If there is anything in a Smriti which contradicts the Sruti, the Smriti is to be rejected.
Manu, Parasara, Yajnavalkya, Gautama, Harita, Yama, Visnu, Sankha, Likhita, Brhaspati, Daksa, Angiras, Pracetas, Samvarta, Acanas, Atri, Apastamba and Satatapa are the eighteen sages who mastered the Vedas with their superhuman power and derived the Smrtis from them. Their works are known after them like Manusmrti, Yajnavalkya-smrti, Parasara-Smrti and so on, and they contain all that we need to know about all the dharmas to be adhered to.
"Dharmo Rakshati Rakshitah" which occurs in the fifteenth verse of the eighth chapter of the Manu Smriti is so meaningful and relevant: "Dharmo Rakshati Rakshitah": who shelters and defends the dharma, the dharma defends and shelters. In Poorva Mimansa Jaimini Rishi declares the samething.
The Yajnavalkya Smriti states:
'The Shruti, the Smriti, the approved usages, that which is agreeable to one's inmost self or good conscience, and has sprung from due deliberation, are ordained as the foundation of Dharma.'