Permanent Live into the Switch
Switched Live out of the switch (to turn the light on and off)
Many switches are marked Com, L1, L2. The usual wiring arrangement is to connect Live to Com and Switched Live to either L1 or L2 it really doesn't matter.
Inside the switch Com connects to L1 and when the switch is operated Com connects to L2 etc. etc.
Peccavi, December 2009