An incandescent bulb is made from a tungsten filament surrounded by a glass bulb from which the air has been removed, so that the filament will not burn up. The material of the filament is a good conductor of electricity. When an electric current is passed through it, the atoms in the filament become agitated, giving off heat and light.
Most rabbis agree that lighting an incandescent bulb is a violation of the prohibition of "burning" (aside from the prohibition of closing an electric circuit on Shabbat). Others see it as being part of the prohibition of "cooking" (see Talmudic Encyclopedia Volume 18, Cooking, 3). Most rabbis agree that lighting a bulb is a Torah violation – of the type of "a metallic ember" mentioned in the Talmud, Shabbat 134a.
Turning off an incandescent bulb is merely a violation of a rabbinical decree (except for some opinions that any breaking of an electric circuit is forbidden because of the prohibition of "disassembly"), since this is "labor whose result is not wanted." From this point of view, turning off a bulb would be forbidden by Torah law only if the person needs the filament itself.
In practice, whenever rabbinical prohibitions are permitted using a technique of gramma (indirect action), it is best to avoid using a device that has an incandescent bulb. In the electric scooter with Shabbat control designed by The Zomet Institute, engaging the control (before Shabbat) disconnects the headlights (if they are not of an LED type) and the turn signals. Of course, if it is needed for an essential function (such as in a security vehicle), gramma will be used to turn the light on if at all possible.