If the starter does not turn the engine although it is not always the fault of of a dead Car Battery the car battery is in good condition, the fault may be a simple mechanical one or it may be an electrical one in the starter-motor circuit. The starter system is simple, and the checks on it are straightforward. Electrical checks are made with a circuit tester or test lamp or with voltmeter . A mechanical check to see if the starter pinion gear is simply jammed in mesh with the engine flywheel can usually be made with a single spanner. It is advised that you also check your starters starting ability and negative amp drawing current at least every 6 month and your local battery clinic will gladly assist you in this matter.
The live terminal on the Car Battery is connected by a heavy lead to a terminal on the solenoid switch which is operated when the ignition switch is turned. The other terminal on the solenoid is connected to a terminal on the starter motor. The second terminal on the motor is earthed via a wire strap through the engine or gearbox and the car bodywork back to the earth terminal on the battery. Modern cars have a pre-engaged starter, which has the solenoid mounted on the casing. Many older cars have an inertia starter, which has a separate solenoid mounted elsewhere in the engine compartment.