Hi Gil:
I'll try, but it's not easy when not using graphics. Heheh.

Suppose there's a device pulling, say 2amps on 220Volts. When putting in a series resistor in one of the leads of 1ohms that would generate a voltage of I*R=V, so 2*1=2volts. If across this resistor an opto-device is connected with an appropriate series resistor you'd get a current running through the opto equivalent to the voltage across the resistor. At the secundairy side capture the voltage into an A/D converter and there you go.
The same goes for measuring voltage, but instead of putting a resistor in series with the circuit, put a resistor of say 5.6Kohms in series with the primairy of the opto. This in parallel with the 220volts would cause about 40mA through the opto led (at 220V).
On the secundairy side connect the collector to a 5V source and the emitter via a resistor of say 1K to ground (experiment with this) and you should have a linear output at the emitter.