DCS- means Distribured Control systemm.This is mainly 
process oriented system.If we ask the feedbeck from the 
system,it will give from the IO field.This is a independent 
control systen.

SCADA-means supervisory control and data acquisition 
system.This is basically a data gathering system and given 
the control based on that datas. This system will depends 
PLC or DCS for its control.

SCADA is basically Remote area used, and its only data 
gathering system and this system will depends on DCS for 
controlling.

DCS is independent system, and this system will get 
feedback and controlling also, SCADA can be added in DCS.

DCS is a system distributed geographically and functionally 
within the process areas. It can view and control the 
porcess based on information received from process field 
instrumentations and can do controlling through the human 
machine interface or through logic built in the DCS 
controllers.

SCADA, it is mainly a system in which the main controller 
exist in the control room (master terminal unit) and 
communicating with remote units (remote terminal units) 
which gathering information from field 
instrumentations.This system also can do controlling 
through logic built in the main controller