Yuga in Hinduism is an epoch or era within a four age cycle. A complete Yuga starts with the Satya Yuga, via Treta Yuga and Dvapara Yuga into a Kali Yuga. We are believed to be in the Kali Yuga (also known as Kalyuga) which is the last age of the cycle. It is believed that the Kali Yuga began after the Mahabharata War ended or after Lord Krishna left the earth.
Satya Yuga is considered to be the first and best Yuga. It is the age of truth and perfection. Caste system is very flexible. Humans are gigantic, powerfully built, handsome, honest, youthful, vigorous, erudite and virtuous. The Vedas are one. There is no agriculture nor mining as the earth yields those riches on its own. Weather is pleasant and everyone is happy. There is no religious sect. There is no disease, decrepitude or fear of anything. Human lifespan is 100,000 years and humans tend to have hundreds or thousands of sons or daughters.
Treta Yuga is considered to be the second Yuga in order, however Treta means the "Third". In this age, virtue diminishes slightly. At the beginning of the age, many emperors rise to dominance and conquer the world. Wars become frequent and weather begins to change to extremes. Oceans and deserts which never existed before are formed. People become slightly diminished compared to their predecessors. Agriculture, labour and mining become existent. Average lifespan of humans decreases to 1000-10,000 years.
Dvapara Yuga is considered to be the third Yuga in order. Dvapara means "two pair" or "after two". In this age, people become tainted with dark qualities and aren't as strong as their ancestors. Diseases become rampant. Humans are discontent and fight each other. Vedas are divided into four parts. People still possess characteristics of youth in old age. Average lifespan of humans is around a few centuries.
Kali Yuga, the final age, is the age of darkness and ignorance. People become sinners and lack virtue. They become slaves to their passions and are barely as powerful as their earliest ancestors in the Satya Yuga. Society falls into disuse and people become liars and hypocrites. Knowledge is lost and scriptures are diminished. The environment is polluted, water and food become scarce. Wealth is heavily diminished. Families become non-existent. Average lifespan of people is only about 100 years.
Hence, it is important to understand the importance of Hinduism through these Yugas and it is believed that we as humans should not lose our virtue and character irrespective of being in the Kali Yuga and must always adhere to Lord Krishna in time of difficulty. It is stated in the Gita that "Although Kali-yuga is an ocean of faults, there is still one good quality about this age: simply by chanting the names of Krishna, one can become free from material bondage and be promoted to the transcendental kingdom."