This article is about the Yugas in Hinduism. For the Indian meditation excercise, go to Yoga.
Hindus believe that there are periods of time called yugas ("ages" or "eras"): Satya Yuga, Treta Yuga, Dvapara Yuga, and Kali Yuga. When Kali Yuga ends, a new Satya Yuga begins.
Satya Yuga (also called Krita Yuga) is the first age, which lasts for 1,728,000 years. It is "the age of truth" or "Golden Age", when all humans are good and very religious. They are ruled by gods and live for many centuries.
Dvapara Yuga is the third age, which lasts for 864,000 years, when humans are still religious but their lives are much shorter. The last Dvapara Yuga ended with the death of Krishna and Mahabharata is set in its final years.
Kali Yuga is the fourth and final age, which lasts for 432,000 years. It is the "dark age" when humans lose their spirituality and are abandoned by gods. Hindus believe that we currently live in a Kali Yuga.