Most modern light switches have 3 terminals marked Com - L1 and L2.
In one position Com is connected to L1 and when the switch is set to the other position Com is connected to L2
If you just need a simple switch (light or lights operated from just one switch position) connect the live input to Com and the live output to either L1 or L2 as you wish - it makes no difference.
Peccavi, May 2009