Where you lock people up.

Why?

Several possible reasons :

  • reform
  • punishment
  • prevent serial criminals criming again

Also, prison or prison-like systems have been used time and time again merely as a mentally-unintensive clearing house for less "desirable" demographics. Why deal with a problem when you can sweep it under a giant rug?

So should we see prisons as a useful solution to a common problem, or are they one of many "cheap" infrastructures that serve mainly as pointers to our failings as a society? Just as poorhouses were testament to an imbalance in equity, is an abundance of prisons testament to a major imbalance in, say, interpersonal upbringing? – GrahamLally

RobertXCringely points out that the prison system is reliably predicted to cause crime : http://www.pbs.org/cringely/pulpit/pulpit20040812.html