Can any body differentiate Practical difference between Sink and Source in PLC

If PLC card supplies power to a field device, it has sourcing I/O. If it it doesn't and field device is powered externally then it has sinking I/O.

To expand on Vladimir's comment:
Assuming 24 vdc I/O cards.
On a transistor output card Sourceing is PNP or Active High. This means that you connect the output to the + side of the load device and connect the other side to 0vdc. If the output Card is Sinking it is NPN or active low meaning that you would tie the + side of the device to 24vdc and the common would connect to the output card.
On input cards if the input card is a sourceing card you would use NPN sensors. The cards common connection would be to a +24vdc source and the device (sensor) would give a path to 0vdc. If the input card was sinking you would use PNP sensors. The sensor would be connected to the + 24vdc source and the output from the device would connect to the PLC input card which would have a common connection to 0vdc.

thanks for the great comments .

Keep in mind the frame of reference of the product vendor, which can cause confusion if not understood up front. For instance, some (most) label their "device" (I/O module, sensor, etc.) according to the internal operation while others label according to the operation of what it connects with.

For sinking type its NPN configuration of field devices are used in which current flows through the sensor/device and the sinking type input card of plc give the current to sensor/device, load is connected between +ve and sensor/device output. Sourcing type its PNP configuration of field device in which current flows from sensor/devices, load is connected between sensor/device output and common or-ve. for this a general input card is used or special cards are to available.

If you want to run two relays with different voltage ratings on the same module you'd need to wire the PLC as sinking. It will allow you to run several voltages ratings by switching their common path to ground which is negative (-). Source will limit your choice of using several voltages on one module.

Simply stated Current Sinking is connecting (switching) ground to the device (load) through as an example a sensor output, and Current Sourcing is connecting (switching) positive DC to the device (load). Banner Engineering has a very valuable "Handbook of Photoelectric Sensing. Take a look.

Can any one explain if i have to controllers working in parallel i want to shuttdown one controller (Say master) gives command to shuttdown the other controller what would be the sink and source relation in this scenario? should the commander be source or sink?