The starter motor is what actually turns the engine to start it; however, turning the key does not directly apply power to the starter motor.
The solenoid operates by an electromagnet to make the physical connection from the starter motor to the engine. When the connection is made, the starter motor turns.
Switch Inside the Solenoid
When the solenoid operates to make the contact between the starter motor and the engine, a switch inside the solenoid connects power from the battery to the starter motor.
Switch to Operate the Solenoid
When you start your car with a key, a switch connects power to the solenoid. This can also be called a starter solenoid switch.
Starter Solenoid Relay
In some cars, turning the key operates a relay that operates the solenoid. This replaces a starter solenoid switch and allows more control points to keep the car from starting unless it is in neutral or park.
When you turn the key to start your car, several things happen in the process of starting the engine. Two of those operations involve a starter solenoid switch.