Switches are just gaps in a circuit where you can choose to close the gap.

If the gap is open, no current can flow.  If the gap is closed, current can flow.

TERMINOLOGY:

• closed circuit: when current can flow  (switch is "on")
• open circuit:  when current cannot flow (switch is "off")
• short circuit: current can flow, but not where we want it to (I hate it when that happens)

SYMBOL: