In the US, it was customary to run power to the fixture (Black = Hot, White = Neutral, bare=ground). In order to switch the light on, a cable containing black, white and bare are run to the switch. Since it switches the black wire, the white wire has to be taped or colored at the end to black on both ends.

The new standard is to put neutral at the box, so you have hot, neutral, ground in and a switched hot out.
This allows for automation devices.

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The reason why you measure full line voltage across the switch, when the switch is off is the lamp is acting as wire. Only with the lam p out of the circuit will the voltage be zero on one end of the switch.

 

And just to make it more confusing, the 2017 National Electrical Code (US) now mandates you pull a neutral to every switch box for "future" use

 

Here in Oz, it is normal to have a red wire as Active and the white wire is the SWITCHED active.

Does that help at all, or just muddy the waters?