PLC stands for the programmable logic controller. DCS stands for Distributed Control System. At one time, the programmable logic controller and the distributed control system had many more differences than they do now.
Now, it is more difficult to see the differences between these two systems. In the programmable logic controller, the whole system with both of its hardware and software can come together to accomplish a task set out before it. In the distributed control system, this system may be given a task, and there is a division that takes place to get the task accomplished. DCS may be better to handle larger systems compared to the PLC. However, the PLC can become a part of a DCS when needed.