The XOR Gate is a redstone circuit block added by the RedPower mod. As the name suggests, it acts like a simple XOR gate for redstone. The output is on when both input states are different. Another way to understand it is as follows: any time either input toggles, the output toggles. If you have two light switches in your house that work the same light, you understand an XOR gate.
When you have two levers in a room both controling the piston that covers/reveals a glowstone block.(Light Switches)
0's represent an inactive signal. 1's represent an active signal. Note that the outcome is equally to the Output of the NOR Gate.