In Hindu mythology, as per the Puranas, there were 4 periods since the beginning of time:
- Satya Yuga - the first 10,000,000 years
- Treta Yuga - the next 1,000,000 years
- Dwapar Yuga - the next 100,000 years
- Kal Yuga - the next 10,000 years
After Kalyug, and after Vishnu takes his Kalki avatar, which would be his 10th incarnation, a new Satya Yuga is supposed to start. Beyond that, nothing is mentioned.
Anyway, Treta Yuga was when you had the Ramayan near the end, and events leading up to it. After Rama's passing, the Dwapar Yuga started, and ended after Dwaraka submerged following the passing of Krishna.
Since Krishna's history and the Mahabharat don't fully co-incide, sometimes the term Dwapar Yuga is used to encapsulate events from both under one head.
