From About Hinduism:-
According to Hindu scriptures, all mortal beings are destined to pass through four great epochs in every cycle of creation and destruction.
The four great epochs in Hinduism are: Satya Yuga, Treta Yuga, Dwapar Yuga and Kali Yuga. Satya Yug or the Age of Truth is said to last for four thousand divine years, Treta Yuga for three thousand, Dwapara Yug for two thousand and Kali Yuga will last for one thousand divine Years that equals to 432,000 earthly years.
Throughout these four yugas, Lord Vishnu is said to incarnate ten times in ten different avatars. During the Age of Truth (Satya Yuga), human beings were spiritually most advanced and had great psychic powers. In the Treta Yuga people still remained righteous and adhered to moral ways of life. Lord Rama of the fabled Ramayana lived in Treta Yuga.
In the Dwapara Yuga, men had lost all knowledge of the intelligence and bliss bodies. Lord Krishna was born in this age. The present Kali Yuga is the most degenerated of the Hindu epochs.
Another theory explains these epochs of time on the basis of the degree of loss of righteousness in the world. It says, during Satya Yuga only truth prevailed (Sanskrit Satya = truth), Treta lost ¼ truth, Dwapar lost ½ truth and Kali is left with only ¼ truth. Evil and dishonesty has replaced truth in the last three ages or yugs.
Wikipedia on the other hand reports:-
Kali Yuga is the last of the four stages that 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.
According to the Surya Siddhanta, an astronomical treatise that forms the basis of all Hindu and Buddhist calendars, Kali Yuga began at midnight (00:00) on 18 February 3102 BCE in the proleptic Julian calendar, or 23 January 3102 BC in the proleptic Gregorian calendar
Veda Online explains in detail what happens during Kali Yuga:-
Hindu astrologers have calculated this to have been 2:27 a.m. on February 18, 3102 BC. This took place some 36 years after Lord Krsna spoke Bhagavad-gita to Arjuna.
The scriptures like SB 12.2 teach that during the 432,000 year age of Kali, humanity deteriorates and falls into barbarism. Humans begin to kill animals for food. They fall under the spell of intoxication. They lose all sexual restraint. Families break up. Women and children are abused and abandoned.
The saints and sages of ancient India describe the people of this age as greedy, ill-behaved, and merciless. In this age, says Srimad-Bhagavatam, merely possessing wealth is considered a sign of good birth, proper behaviour, and fine qualities. Law and justice are determined by one’s prestige and power
Severe droughts and plagues are everywhere. Slovenliness, illness, hunger and fear spread. Nations are continually at war with one another.
The men of Kali-yuga seek only money. Only the richest have power. People without money are their slaves.
Now the question arises – when does Kali Yuga end?
There seem to be many different interpretations of the end on different websites. Wikipedia simply mentions it as “When flowers will beget within flowers, and fruits within fruits, then will the Yuga come to an end. And the clouds will pour rain unseasonably when the end of the Yuga approaches”
Interestingly it is also mentioned that “since only 5,000 years is passed from the beginning of Kali-yuga, there is still 427,000 left for the end of Kali-yuga. Even the Kali-yuga is not the end. After the end of Kali-yuga, again Satya yuga starts”
The last statement is interesting since we take into account the predicted golden years that may start in 2012, then it may be true that there are another 427,000 years to go before the end of Kali Yuga. And the golden years within Kali Yuga is expected to last for 10,000 years.
2012 will be an interesting year if all the ancient predictions fall in place.