More than 1 in 100 Americans are in jail, which translates to more than 2.3 million people behind bars in the U.S. With that many people in jail the U.S. leads the world in both the number of people it has incarcerated (more than China, whose overall population is much higher) and it has a higher percentage of its population behind bars than any other country.
The state governments in the United States spend nearly $50 billion every year on prisons, and the federal government spends another $5 billion. The United States has less than 5 percent of the world's population, but it has almost a QUARTER of the world's prisoners.
(Sources: 1, 2, 3)
These are kind of depressing statistics, so here's a slightly (barely) relevant video that might cheer you up: