scada and PLC

http://en.wikipedia.org/wiki/SCADA

http://en.wikipedia.org/wiki/Programmable_logic_controller

scada = supervisory control annd data aquisition. SCADA is the software and communications that allows you to extract information from a PLC or feed infornation into a PLC. The scada also include the HMI human machine interface.

IF you combine a scada and PLC you would call it a DCS.