Differentiate PLC vs DCS.?

The two really don't compare as a PLC is Programmable Logic Controller and DCS is Distributed Control System. Generally, PLCs are stand alone and perform a particular task, where a DSC is a network of PLCs/ RTUs that communicate in some fashion to accomplish a particular task. For example, in a water filtration plant, there might be a PLC that is used to perform a backwash of a particular filter, in that same water plant a DCS may be communicating with 14 filter PLCs and starting the backwash routine when required. . PLC only handled sequential process than DCS can handle both continue process and large loop control and If we see from security angle, PLC doesn't have dongle so people can crack the software easy. DCS have a dongle so it's only license to industry which have it. 

PLCs are parts of a either DCS or SCADA system, so that the question should be DCS vs. SCADA rather than DCS vs. PLC. It is like comparing oranges versus apples but the differentiators are factors like Control, Communication, No. of I/Os that can be connected, Scanning time, History and MMI.