According to the descriptions given in the epics and the purāṇas, a yuga is a colossal unit of time. (See YUGA for more details.)
Out of the four yugas normally described, the Tretāyuga is the second in the series. It has been called ‘tretā’ (= three) since it is three times longer in duration than the Kaliyuga, the fourth and the last in the series.
It lasts for 3600 divine years or 12,96,000 human years!
Compared to the Kṛtayuga, the first, where dharma or righteousness was fully manifested, in this yuga it was less by a quarter.
In this yuga, people were devoted to truth and performed their duties properly. The religion of sacrifices—yajñas or yāgas—got manifested in this age for the first time. Hence they became extremely popular.