Yuga (Devanāgari: युग) in Hindu philosophy is the name of an 'epoch' or 'era' within a cycle of four ages. These are the Saty a Yuga, the Treta Yuga, the Dvapara Yuga, and finally the Kali Yuga. According to Hindu cosmology, life in the universe is created and destroyed once every 4.1 to 8.2 billion years,which is one full day (day and night) for Brahma. The lifetime of a Brahma himself may be 311 trillion and 40 billion years. The cycles are said to repeat like the seasons, waxing and waning within a greater time-cycle of the creation and destruction of the universe. Like Summer, Spring, Winter and Autumn, each yuga involves stages or gradual changes which the earth and the consciousness of mankind goes through as a whole. A complete yuga cycle from a high Golden Age of enlightenment to a Dark Age and back again is said to be caused by the solar system's motion around another star.
The Four Yugas
The Satya Yuga (Devanagari: सत्य युग), also called Sat Yuga, Krta Yuga and Krita Yuga in Hinduism, is the "Yuga (Age or Era) of Truth", when mankind is governed by gods, and every manifestation or work is close to the purest ideal and mankind will allow intrinsic goodness to rule supreme. It is sometimes referred to as the "Golden Age." The average lifespan in the satya yuga is 100,000 years. The goddess Dharma (depicted in the form of cow), which symbolises morality, stood on all four legs during this period. Later in the Treta Yuga it would become three, and two in the later Dvapara Yuga. Currently, in the immoral age of Kali, it stands on one leg.
Dvapara Yuga or Dwapara Yuga(Devanagari: द्वापर युग) is the third out of four yugas, or ages, described in the scriptures of Hinduism. This yuga comes after Treta Yuga and is followed by Kali Yuga. According to the Puranas this yuga ended at the moment when Krishna returned to his eternal abode of Vaikuntha. According to the Bhagavata Purana, the Dvapara Yuga lasts 864,000 years.
There are only two pillars left of religion in the Dvapara Yuga: Compassion and Truthfulness. Lord Vishnu assumes the colour yellow and the Vedas are categorized into four parts that is Rig, Sama, Yajur and Atharva. During these times the Brahmins are knowledgeable of two, sometimes three Vedas, but rarely have studied all the four Vedas thoroughly. Accordingly, because of this categorization, different actions and activities come into existence.
Treta Yuga (Devanagari: त्रेता युग) is the second out of four yugas, or ages of mankind, in the religion of Hinduism, and follows the Satya Yuga of perfect morality and precedes the Dvapara Yuga. The most famous events in this yuga were Lord Vishnu's fifth, sixth and seventh incarnations as Vamana, Parashurama and Ramachandra respectively. The Dharma bull, which symbolises morality, stood on three legs during this period. It had all four in the Satya Yuga and two in the later Dvapara Yuga. Currently, in the immoral age of Kali, it stands on one leg. The average lifespan of the Treta Yuga is 1,296,000 years.
Kali Yuga (Devanāgarī: कलियुग [kəli juɡə], lit. "age of [the demon] Kali", or "age of vice") is the last of the four stages the world goes through as part of the cycle of yugas described in the Indian scriptures. The other ages are Satya Yuga, Treta Yuga and Dvapara Yuga.
Kali Yuga is associated with the apocalyptic demon Kali, not to be confused with the goddess Kālī (read as Kaalee) (these are unrelated words in the Sanskrit language). The "Kali" of Kali Yuga means "strife", "discord", "quarrel" or "contention".