What is Yuga?

The term "Yuga" relates to the concept that the Earth and its people,
according to most Hindu scripture are living in the Kali-Yuga*,
the most negative of 4 ages in the life cycle of the universe.
A Yuga is described as the out-breath of God where the universe is expanding and growing
followed by an in-breath where the entire universe folds back into its center.  
Due to the increased challenge of the Kali-Yuga also called the Iron or Dark Age,
being the most negative and energy deficient time,
certain Tantra practices of non-dualistic living evolved to help man thrive.

The Hindu’s poetically described how man would decline in this way:

They compared morality to the legs of a bull. In the first of the Yugas, The Golden Age,
the Satya Yuga the bull had four legs, and in each age,
morality reduced by a quarter. Next came the Silver Age and then the Bronze Age. 
By the age of Kali**, morality is reduced to only a quarter of the golden age.
Thus, the bull, the morality of men will only have one leg and be severely degenerated. 
 

*. The most common interpretation is that the Kali Yuga began at midnight on February 18 3102 BCE in the proleptic Gregorian calendar. 
Many Hindu’s consider that this is the time Krishna left his physical body. It is a Hindu belief that avatars don’t die.

**Kali Yuga should not be confused with the goddess Kali. The "kali" of Kali Yuga means "terible,"
whereas the goddess "Kali" is the feminine form of "Kala," which means "time."
Though it is believed that Kali is the one that governs this age.