The key ignition of an automobile serves two purposes: It ensures that only the person who carries the keys can start the car and drive it away, and it not only starts the engine but activates all the other electronic and mechanical systems in the vehicle. The same key also often gives access to the trunk and perhaps even a lockable glove box.
While some cars now use keyless ignition, car keys are still used for a majority of the new cars in the world. Keys are now usually symmetrical, so that drivers can insert them either way--unlike most house keys, for example.
As the key slides into the slot, it pushes a set of tumblers in the ignition into a precise pattern to allow the key to be rotated to close the ignition circuit. That circuit then delivers power to the starter motor, which turns the engine so the spark plugs can fire and the engine operates.
Some newer cars add coded signals from the fob to the ignition as well, or use those as a substitute for the metal key shaft with notches or other patterns on it. Other variations on ignition keys include the valet key--which operates only the ignition, so the car can be moved, but not the trunk or the glove compartment, and may even limit the car's power output. There are also trunk-only keys, which cannot start the car.