What is the difference between SCADA, DCS and PLC?

SCADA is a software where you can monitor and control the plant.

PLC is control system where you can program and control your action.

DCS has both hardware and software but it is a little slower than PLC 

Scada is Supervisory control and Data acquisition. It means its only a software we can monitor and control the plant . For taking the controlling action   SCADA need hardware either its a PLC or RTU.

RTU Mainly using remote location ie in pipe lines and Wellhead platforms etc.

PLC we are using in critical action and centralized control. Where scan time is very less.

DCS is Distributed Control system. DCS have hardware and software. DCS doing control action distributively.