What is difference between SCADA and DCS?

SCADA- It means Supervisory Control And Data Acquisition and refers to centralized systems which monitor and control sites spread over large areas (may be miles or kilometers).It mainly consists of three components, RTU(Remote Terminal Unit), Master Station and Human Machine Interface (HMI) and the third one is communication Infrastructure. Individual sites are run independently through RTU. These RTUs collect and send the status data to the Master Unit. The operator at the Master Station, with the help of MHI (computer),can change the set points or give commands to these sites.
DCS- It means Distributed Control System which is a term used in variety of industries to monitor or control distributed equipment. Input and Output modules form component parts of DCS. These modules receive and give signal to input and output instruments respectively. Elements of a distributed control system may directly connect to physical equipment such as switches, pumps and valves.DCS are dedicated systems used in industries such as oil refining, petrochemicals, central station power generation, pharmaceuticals, food & beverage manufacturing, cement production, steelmaking, and papermaking.

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 governor 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