DCS or Data Control System and SCADA or Supervisory Control and Data Acquisition are both similar in terms of function, but their differences start when you consider the general design of the two, and how both are used to control processes. DCS is most concerned with processes i.e., it focuses more on every process that constitutes an operation.
SCADA, on the other hand, works by collating and providing data relating to the personnel that is controlling the operation. Tasks that are involved when you use DCS are done in sequential order, and it does not work by focusing more on the events. These events can only be recorded if they are scanned by the station.
In contrast, actions are triggered with SCADA only if there is a change in any of the components of the operation. DCS is used mostly for operations in a very small area or locale, whereas SCADA is a better choice for a very wide geographical location.