What's the difference between DCS and SCADA systems?

If an RTU forms a single unit, then SCADA systems are the whole networked based system. Similar to DCS, a network of RT-controllers and computer workstations form the control network, RTU here compose the SCADA system along with supervisory workstations located at a far-away site. Here, two confusions may emerge: First, the term SCADA has been used to describe one level in the industrial automation PCS architecture, which DCS derives as a part of it, various times. Hoverer, here the name is slightly different, SCADA system, which means that the whole architecture is supervisory monitoring and control based, not for process control purposes. The second query that may arise regarding SCADA systems is what would distinguish them from DCSs if both are technically networked control units and HMI workstations. The answer to this enquiry from a more technical perspective can be summarized in the following points: 

 First, although both are made by networking, SCADA systems are called networked based in industry while DCS are not. This is because of the fact that even though it is distributed, a single DCS system should be located geographically at the same process plant site while a SCADA system may communicate to several remote sites located at different locations. 

Second, as can be demonstrated since they connect remote sites to control rooms that require longer distance ranges, SCADA systems are WAN based while as presented DCS are LAN ones. Third, as stated above SCADA/RTU systems are PLC based whereas DCS are regular RT-controller processor units. Lastly, given that SCADA systems are mainly PLC based, it is settled that the latter are event driven while the DCS is time determined. 

DCS is process oriented, while SCADA is data acquisition oriented, DCS is process state driven, while SCADA is event driven, DCS is commonly used to handle operations on a single locale, while SCADA is preferred for applications that are spread over a wide geographic location, DCS operator stations are always connected to its I/O, while SCADA is expected to operate despite failure of field communications.

In addition to the above answer, scada was conventionally used for smaller applications like an industry with 2/3 process plants where as Dcs is an overall umbrella control system for numerous such plants, it can  also include various scada modules by different vendors for individual plants.

DCS (Distributed control systems) is process oriented. A DCS is a process control system that uses a network to interconnect sensors, controllers, operator terminals and actuators. A DCS typically contains one or more computers for control and mostly use both proprietary interconnections and protocols for communications.

SCADA(Supervisory Control And Data Acquisition) is data-gathering oriented. SCADA may be called Human-Machine Interface (HMI). SCADA systems are used to monitor or to control chemical, physical or transport processes.

SCADA is used with both DCS and PLC SYSTEMS.(AND WITH NEWER REPLACEMENT FOR OLD COSTLY DCS,THE PAC)PAC REPLACE DCS AND PLCs.

scada more bigger than dcs.

SCADA is used with both DCS and PLC systems. SCADA includes "Data Acquisition" in addition to "Control". DCS, on the other hand, contains only "Control".

DCS devices did most of the detail work and simply reported to (and took high-level orders from) the SCADA system.

That is just like asking what is the difference between a car and a BMW. SCADA is the name for a complete system designed to supervise, control, monitor and gather data whereas DCS is a tool that integrates various control systems in a SCADA

DCS and SCADA seem very similar and intertwined, but in sheer concept they are different in purpose. In simple terms, DCS is a control system that is directly involved in the manufacturing process. The alternative to a DCS is a PLC system, which is in principle pre-programmed to certain control functions that are meant to be fixed and unchanging. 

A SCADA can actually be identified by its definition: Supervisory Control And Data Acquisition. It is not a control system, but acts like a supervisor monitoring the control system who is logging input from the field, and only taking action against specific events. In that situation, the Supervisor is actually interrupting the programmed control system to respond to the event. 

A SCADA system can be used with either DCS or PLC control system, but with the current advances in the industry these systems are starting to merge into one solution. 

DCS is process oriented, while SCADA is data acquisition oriented. The SCADA can be views in both DCS & PLC. The DCS is a control device that has higher capabilities compared to an ordinary PLC. On the other hand, SCADA - supervisory control and data acquisition - is mainly a software environment that can communicate with PLC's and DCSs also for that matter in order to monitor the functionality of those devices and perform data acquisition and data logging to a monitoring platform - say a computer in a control room etc.

Conventionally SCADA was more for data-gathering where as DCS was for controlled processes.

DCS is process oriented: it looks at the controlled process (the chemical plant or whatever) as the center of the universe, and it presents data to operators as part of its job. SCADA is data-gathering oriented: the control center and operators are the center of its universe. The remote equipment is merely there to collect the data--though it may also do some very complex process control

A DCS operator station is normally intimately connected with its I/O (through local wiring, FieldBus, networks, etc.). When the DCS operator wants to see information he usually makes a request directly to the field I/O and gets a response. Field events can directly interrupt the system and advise the operator. SCADA must operate reasonably when field communications have failed. The 'quality' of the data shown to the operator is an important facet of SCADA system operation. SCADA systems often provide special 'event' processing mechanisms to handle conditions that occur between data acquisition periods.

SCADA system is event driven, while a DCS is process state driven. A DCS is primarily interested in process trends, a SCADA system in process events 

There are many other differences, but they tend to involve a lot of detail.

SCADA generally supports storing of database where as DCS generally does not support database. In recent times due to technology advancement features of the two are getting blended and both on foreground appears similar.