What are the two major differences between DCS and SCADA systems?

An automation system within a campus is usually refferred to as a DCS, while SCADA systems comprise of different stations disributed over large geographical areas.

DCS can be deployed for the supervision of a cretain process, however if a number of DCS systems at different locations are interconned to a master station, they form a SCADA system.

I have already given this answer, in todays' world DCS and SCADA are crossing their lines.

I am presently using Honeywell DCS C200E, it also have SCADA points to solve the long distance control.

DCS is using FTE protocols to communicate with its field servers and controllers. It has many designated cable pairs according to distance. But DCS is single controller itself. But you can extend its capabilities by adding small PLCs or RTUs.

SCADA is used when a small controllers at different area/processes are required like RTUs. It is just supervisory controller used to control the controllers and acquire real-time data.

in scada all rtu's are connected by mtu but in dcs every rtu can be individual

SCADA is most used for wide area , e.g.10 km and more, 

DCS is used for a specific area within a specific boundry, 

DCS ; distributed control systems is usefuul in big industrial operation ,  one error can not stop all the  system ,and the operation time is fastest than SCADA because DCS connecting  many

plc without any change in the media 

PLC is a programmable Logic controller which is used mainly for interlocking different equipments.DCS is Distributed Control Systems which is used to control very big plants by using simple GUI screens.PLCs are interfaced with DCS for interlocking.SCADA is Supervisory Control and Data Aquisition which is used to control and Monitor small number of Equipments in a field.SCADA systems are typically used to perform data collection and control at the supervisory level. Some systems are called SCADA despite only performing data acquisition and not control.The supervisory control system is a system that is placed on top of a real-time control system to control a process that is external to the SCADA system (i.e. a computer, by itself, is not a SCADA system even though it controls its own power consumption and cooling). This implies that the system is not critical to control the process in real time, as there is a separate or integrated real-time automated control system that can respond quickly enough to compensate for process changes within the time constants of the process. The process can be industrial, infrastructure or facility.

            DCS                                VS                                        PLC

1.   Single vendor                                                                    Mutiple

2.    Small Area /LAN                                                            Large Area/WAN

3.    Many I/O                                                                         Less I/O

4.  Use for Monitoring                                                       Used for Controlling

Distributed Automation system within a building/campus/Industry is a clear DCS and SCADA is the system mastering many DCS stations lying on Tele/long distance, like communication and telecommunication.

From my point of view the main different is the DCS has almost one data base

but Scada have more one DB.

1. 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.
2. DCS is process state driven, while SCADA is event driven. For this, we can used SCADA as sub-system for DCS.