What is the difference between Current controlled device and voltage controlled devices ? on which basis they are differentiated ?

Current controlled devices are those, whose output characteristic depends on input current

Voltage controlled devices are those whose output depend on input voltage.......

We have to see the output of the device and examine weather its output depend on input current or input voltage.......................Thanks.

If output of a device depends on input current, it is current controlled device coz input current controls output.

If Output of a device depends on input voltage, it is voltage controlled device coz input voltage controls output.

In other words if the output variations are due to input current variations, then this device is current controlled and if the output variations are due to input voltage variations, then the device is voltage controlled.

Like BJT is current controlled and FET is voltage controlled. 

Current control ussualy use4-20 mA as a control signal.

Voltage control ussually use1-5 VDC as a control signal.

which one we use ? that's depend on final element signal requirement.

for example there are control valve actuator (final element) need current or voltage as control signal.

You should select output modul controller as per final element signal requirement.

If your controller output is not same as your final element signal requirement, you must use current to voltage converter OR you can use250 Ohm Resistor. 

Another issue that should be consider is drop voltage, drop voltage issue can be solve with correct cable/wire sellection and good grounding/earthing  

I agree with all above friends answers

 current controlled devices control in out put current  

Voltage  controlled devices control in out put Voltage

as an example we take single phasing preventer,the spp is in voltage controlled and current controlled,i they are differentied with load.

current controlled device is dealing whith4 to20mA signal that will make its operation example of this is a certain positioner sipart positioner in alborg boiler driving the forced draft fan. voltage controlled device is controlled by voltage usually1 to5V signal to make it operate or move...example is a hydraulic valve actuator solenoid which is used to controlle the speed of a certain pump in framo system.