There are several interpretations of the four different yugs in our scriptures. I am briefly outlining only one here.
In Satyug, virtue reigns supreme and is depicted by the Dharma (that which upholds, supports or maintains the regulatory order of the universe) bull standing on all four legs of righteousness.
These four ages, ranging from a high golden age of enlightenment to a dark age of misery follow a timeline ratio of 4:3:2:1, where Satyug comprises of 1,728,000 years, Tretayug of 1,296,000 years, Dwaparyug of 864,000 years and Kalyug of 432,000 years. Scholars say we are 5000 years into the Kalyug age which leaves us with 427, 000 years to go before Lord Vishnu shows up in the avatar of Kalki leading to the end of the age.
The main reason is to destroy the wicked, to restart the slate of creation anew and to restore the purity of conduct in people’s lives. This avatar will come riding hard on a white horse with a drawn sword blazing like a comet- hence the speculation by many that asteroids will smash into the earth tolling the death knell for us all. Many Hollywood movies have also been made depicting this style of Ragnarok (meaning ‘Doom of the Gods’ in Norse mythology), including one starring Bruce Willis, Ben Affleck and Liv Tyler, aptly named ‘Armageddon’!