It is basically slavery when you get someone's labor without paying them for it.

Here are some interesting things to read if you want to know more.

Good general overview of prison labor:

http://thesocietypages.org/socimages/2013/04/04/prison-labor...

More detailed account of prison labor in Nevada regarding casinos and construction.

http://vltp.net/casinos-prison-labor-strange-bedfellows/

The slave market:

http://www.unicor.gov/

You can even get slave labor to operate your call center:

http://www.unicor.gov/services/contact_helpdesk/