What? A high incarceration rate.  
The U.S. has the highest number of prisoners in the world. Both as a percentage of our population and in total numbers, we incarcerate more people than any other country—more than China, Iran, Russia, India, North Korea, South Africa. A major increase in prisoners—from about 300,000 to more than 2-million—has happened in the past thirty years.[1]
What For? For non-violent drug offenses.
Almost half of all people locked up in state prisons are there for non-violent offenses. Almost 25% of the incarcerated populations are there for drug offenses—parole violations extend or inflate minor offenses. Three-strikes-you’re-out policies transform petty drug crimes into life-in-prison.    
Who? Communities of Color.
This increase in prison population has been comprised disproportionately of people of color. Even though white people engage in drug offenses at slightly higher rates, they are punished much less frequently and severely. As this graphic illustrates, only 1 in 106 white males are incarcerated while 1 in 15 black males and 1 in 26 Hispanic males currently live behind bars.
How? Systems of Racial Injustice
How could this come about in a post civil-rights era of colorblindness, in a time when Obama is president, and overt racism is largely scorned? In her new book, Michelle Alexander details the many entry points into this form of racial injustice: school-to-prison pipelines, police incentive to prosecute a war on drugs, targeting of poor communities, bad legal representation, trumped up felony charges, and unfair sentencing practices. All combined, these practices lead to a form of racial injustice that most of us can’t see, but has profound consequences.
First off, prisoners and felons experience increased barriers to voting.  This results in masses of racially disenfranchised people reminiscent of Jim Crow-style obstruction to the polls. Secondly, opportunistic corporations have seen profit potential in the exploding incarceration rate. Privatized prison companies are increasingly winning government contracts. A profit motive for incarceration may not be illegal, but strikes me as inherently immoral.

[1] Alexander, Michelle. 2010. The New Jim Crow: Mass Incarceration in the Age of Colorblindness. New York: The New Press.