Car - Parts of a Car

What is the ignition switch?

The ignition switch is the master switch that provides power for the vehicle's electrical accessories, computer, fuel and ignition systems. It also routes current from the battery to the starter to crank the engine.

An ignition switch has four positions:
OFF - Or LOCK position, Turns off power to the engine and electrical accessories
ACC - Accessory position that provides power to electrical accessories only, not the engine. Electrical accessories may include radio/entertainment system, electric windows and sunroof. This is mainly for passenger convenience but prolonged use of any of these without the motor running will drain the battery.
RUN - The "ON" position that provides power to the engine and electrical accessories. The switch must be in this position for the engine to run and for the vehicle to be driven.
START - Used only to start the engine.