A SCADA system is a system consisting of a number of remote terminal units (RTUS) collecting field data connected back to a master station , the master station displays the acquired data and also allow the operator to perform remote control tasks

In DCS the data acquisition and control functions are performed by a number of distributed microprocessor based units situated near to the devices being controlled or the instruments from which data is being gathered

simply the difference is scada collects data from process and equipments to display it in master station to the operator and also allow the operator to perform remote control tasks

DCS collects data from processes to control process (these process are related to each other like in power plant (like the process of opening or closing the steam governer depends on the power demand from the generator) so u can't use a single controller u want a number of controllers all of them are communicated with each other) and also display these data in master station