The Four Yugas

Each yuga is an age with specific characteristics in which incarnations of Krsna appear. The four yugas make up a cycle called divya-yuga, which lasts 4,320,000 years. One thousand of these yugas equal one day of Brahma, which is called a kalpa. Brahma's lifespan is 100 years of his time. In each yuga there is a specific process of self-realization (yuga dharma).

SATYA YUGA - (sometimes also called krta-yuga): the golden age lasts 1,728,000 years. The process of self-realization in this yuga is meditation on Visnu. During this yuga the majority of the population is situated in the mode of goodness and the average life span at the beginning of the yuga is 100,000 years.

TRETA YUGA - also called the silver age, lasts 1,296,000 years and the process of self realization is the performance of opulent yajnas(sacrifices). The average life span is 10,000 years and the godly qualities decrease one fourth compared to the Satya yuga.
It is during this age that Varna-asrama-dharma is introduced.

DVAPARA YUGA - or the bronze age, lasts 864,000 years and the process of self realization is the worship of the deities within temples. Godly qualities are reduced to 50% by now and the average life expectancy is only 1000 years.

KALI YUGA - the iron age of hypocrisy and quarrel lasts 432,000 years. Lord Krsna appeared in His original, trancendental form right before the beginning of Kali Yuga. The process of self realization is sankirtana, the chanting of the Holy Names of the Lord. God consciousness is reduced to 25% of the population and life expectancy is only 100 years. By now already 5000 years of Kali yuga have passed and it is predicted that by the end of the yuga people will hardly be older than 20 years and their only food will be meat.