A starter motor requires very high current to crank the engine, that's why it's connected to the battery with large cables (see lower diagram).
The negative (ground) cable connects "-" battery terminal to the engine block close to the starter.
The positive cable connects "+" battery terminal to the starter solenoid.
The starter solenoid works as an electric switch - when actuated, it closes the circuit and connects the starter motor to the battery. At the same time, it pushes the starter gear forward to mesh with the engine's flywheel.
How the starting system works
When you turn the ignition key to the "Start" position, the battery voltage goes through the starter control circuit and activates the starter solenoid, which in turn energizes the starter motor. The starter motor cranks the engine.
A starter can only be operated when the automatic transmission shifter is in "Park" or "Neutral" position or if the car has a manual transmission, when the clutch pedal is depressed.
To accomplish this, there is a Neutral safety switch installed at the automatic transmission, (or at the clutch pedal).
When the automatic transmission is not in "Park" or "Neutral" (or when the clutch pedal is not depressed), the neutral safety switch is open and the starter relay disconnects the starter control circuit.
Car starting problems
If when you turn the key to the "Start" position, you hear the starter cranking as usually, but the car doesn't start, then the problem is most likely not with the starting system.
If when you turn the ignition key to the "Start" position and nothing happens, or all you hear is just a click, or the starter cranks very slow then you probably have a problem with one of the component of the starting system. Most often, it's the battery or the starter motor itself.