There are several power plants and manufacturing facilities in my area that uses DCS. Generally, it is a system that connects with other systems to coordinate processes while communicating with maintenance, inventory, accounting, etc.
The ones that I've seen and worked with may have as few as one monitor upto 6 or 8 monitors. It was the system used to start up, run, and shutdown chemical and mechanical processes, while still maintaining communication to other systems for control. It was used to control anything from automatic valves, pumps, all kinds of equipment, and even other computers.
This is an introduction of sorts from one of the companies used in my area: http://www.reliance.com/pdf_elements/j2629.pdf
Another interesting site: http://www.ie.psu.edu/people/faculty...on/distcon.htm
How it works is a huge question. Or I guess it could be huge.