The four yugas
The four yugas or ages are described in the Bhagavad Gita (8.17), the Srimad Bhagavatam (3.11.19), the Vana parva of the Mahabharatha, amongst other places. In the Satya or Krita yuga, which lasts 1.728 million years, there is wisdom, religion, virtue with no vice and ignorance. All the four legs of religion - truthfulness, austerity, mercy and charity are completely manifest in the people of this age, who live in the mode of goodness with life spans of 100,000 years.
In the Treta yuga, which lasts for 1.296 million years, there is a 25 percent reduction in religiosity. People still follow the Vedas but more so for spiritual and economic gains, than for pleasing the Lord. The formerly classless society is now divided by the Varanashrama and the influence of irreligion is felt through lying and violence. People in this age live in a mixed mode of goodness and passion with life spans of 10,000 years.
In the Dvapara yuga people are interested in fame, glory and nobility. They are opulent with large families and live upto 1000 years. In the this yuga, which lasts for 864,000 years, life starts becoming arduous as wars, diseases, passion and hatred begun to grow. There is a further 25 percent reduction in religiosity and people are mostly in the mode of passion.
In the Kali yuga, which lasts for 432,000 years, people live for upto 100 years, mostly in the mode of ignorance, and of the four legs of religion, only truthfulness remains to some extend. With a further 25 percent reduction in religiosity, people are mostly engaged in irreligious practices solely for sense gratification. People in the age of Kali will be lazy, quarrelsome, greedy and deceitful.
In the Srimad Bhagavatam it is said that the age of Kali began five thousand years ago when Sri Krishna left the planet. It began in earnest when Srngi the immature Brahman son of Sage Samika, cursed Maharaja Parikshit to die in seven days. This fall of the Brahmans, who are known for their mercy and forgiveness, then spread to the ruling class, and the rulers became vain, proud and selfish. It then spread to the merchant class who became greedy and dishonest. In this way the age of Kali became an age of the sudras, of people without any religious knowledge, living for the sake of sense enjoyment, forever angry and discontent.
The time period of the four yugas is called a divya-yuga which lasts for 4.32 million years, and repeats itself cyclically. After every Kali yuga, a new cycle begins with Satya yuga. It is the tenth incarnation of Sri Krishna, Lord Kalki, who personally descends at the end of each Kali yuga to kill by the millions the miscreants who are envious of the Supreme Personality of Godhead and set the stage for Satya yuga of the next cycle to manifest.
The golden age of Kali
The Srimad Bhagavatam (11.5.38-40) talks about a period in Kali yuga where even demigods and denizens of the higher planetary systems desire to take birth. This period, which started about 500 years ago with the appearance of Chaitanya Mahaprabhu, will last for 10,000 years. It is said that in this period the influences of the Kali yuga will be counteracted by the pure devotees of the Lord. Their weapon will be the nama-sankirtana or devotional chanting of the name, and with this they will reverse the influences of Kali yuga.
Those people who are fortunate to come in contact with these devotees, and take up devotional service themselves will find it very easy to go back to Godhead. What is achieved by austerities in the Satya yuga, meditation in the Treta yuga, elaborate temple worship in the Dvapara yuga is obtained by chanting the name of the Lord in the Kali yuga. This is a special benediction provided to the people of this age, since the environment is so polluted and the potency so diminished. Simply by chanting the name of the Lord one can achieve the highest form of liberation.
Once the golden age of Kali comes to an end, irreligion will set in fully. People with religious propensity will be hunted like animals, and they will be forced to leave the cities and live in the mountains and forests.
Activities of Lord Kalki
Lord Kalki will appear in the home of the most prominent Brahman, Vishnuyasha, in the village of Shambhala. The Padma Purana (6.242.8-12) states that Vishnuyasha is actually an incarnation of Svayambhuva Manu, who performed great austerities to have the Lord as his child. He received the benediction that the Lord will appear as his son three times. Thus Svayambhuva Manu appears as Dasaratha (Rama), Vasudeva (Krishna) and finally as Vishnuyasha (Kalki).
When Lord Kalki appears, people will be too strongly in the mode of ignorance to even comprehend the basic concepts of religiosity. So Lord Kalki does not come to teach, but simply to chastise, punish and cleanse the planet. The Srimad Bhagavatam (12.2.19-20) states that Lord Kalki, the Lord of the universe, will mount his swift white horse, Devadatta, and exhibiting eight mystic opulence, He will travel far and wide with His army of devotees to kill the sinful and faithless.
Only those who are engaged in the worship of the Supreme Lord will be left. Once the planet has been rid of the evil, the spirituality in these devotees will be heightened by the fragrant breeze carrying the smell of the sandalwood paste from the Lordís body. In this way, Lord Kalki will reestablish righteousness on the Earth and in the minds of His devotees and then go back to His eternal abode.
The Srimad Bhagavatam (12.2.37-38) and the Vishnu Purana (4.24) states that there are two persons who have been patiently waiting for thousands of years for the descent of Lord Kalki. Devapi of the race of Puru and brother of King Shantanu, and Maru, a descendant of King Ikshvaku. Sustained by the mystic strength obtained from devotion they have lived through the yugas in a village named Kalapa, hidden from normal vision. Then at the instruction of Lord Kalki they will return to the society to become great kings and establish kingdoms centered around the sanatana dharma, the eternal service of man to God.