PLCs (or Programmable Logic Controllers) are widely used in factories, and the industrial sector.
These babies control the way the industrial process goes, they can open a valve when a button is pushed some where, they can start cooling when there is over heat, almost any thing if they were well programmed.
Even though there are more advanced versions of these PLC's such as PAC (Programmable Automation Controllers), PLC's are still somehow dominant in the industrial market.
PLCS.net is a web site aiming to teach people even without proper background how PLC's work.
Check it out, hope it is of use to you :)