In Hindu religion, Dharma Shastras are social laws that include the codes of conduct to be observed by individuals living in a society. Dharma Shastras are works by various sages of ancient India . The most famous and the most controversial Dharma Shastra is the Manu Smriti. Apart from Dharma Shastra of Manu, there are 18 Dharma Shastras that are applicable in different periods of time.
A particular Dharma Shastra is applicable only during a Manvantara. A manvantara is the period of time or duration that a Manu (the archetypal human being) rules the entire creation. One Manvantara is 306,720,000 human years.
Apart from Manu, the other Dharma Shastras that are applicable in different periods are named after Apastamba, Bharadvaja, Daksha, Devala, Samakha, Vasishta, Gautama, Samavarata, Vishnu, Harita, Shatanika, Vyasa, Likhita, Shatotraya, Yajnavalkya, Parashara, Sahunaka and Yama.
(Reference: Hinduism Frequently Asked Questions by Chinmaya Mission)