Deflection instruments measure the deviation of a physical quantity (e.g. voltage, current, force) from its zero value, while null output instruments determine the exact value of the physical quantity by nullifying it.
In other words, deflection instruments display the magnitude of a physical quantity, while null output instruments determine its precise value.
Examples of deflection instruments are analog voltmeters and ammeters, which measure the deviation of a voltage or current from zero. Examples of null output instruments are digital multimeters, which determine the precise value of voltage, current, or resistance by nullifying it.