what is the difference between DCS and SCADA?

Answer 1

Lets say you have a factory with many different machines. Each machine would have its own PLC or Programmable Logical Controller. Each PLC would be wired and programmed to take care of its own area of the factory. This is called a Distributed Control System.

SCADA means Supervisory Control And Data Acquisition system. It brings all of the information together in one HMI or Human Machine Interface by talking to each of the distributed PLCs. An operator uses the SCADA to control, monitor and document the overall process.