Apart from the main charging, starting and ignition circuits, there are other circuits that power lights, electric motors, the sensors and gauges of electrical instruments, heating elements, magnetically operated locks, the radio and so on. All Circuits are opened and closed either by switches or by relays - remote switches operated by electromagnets.
The strength of the current is measured in amperes (amps); the pressure that drives it round the circuit is called voltage (volts). Modern cars have a 12 volt battery. Its capacity is measured in amp/hours. A 56 amp/hour battery should be able to deliver a current of 1 amp for 56 hours, or 2 amps for 28 hours.
The other, probably rated at about 20-30 amps, protects components not wired through the ignition — horns, interior lights and the cigarette lighter. Where a single fuse protects a number of circuits and keeps blowing, each circuit must be checked individually to discover which one is faulty.
In a negative (-) earth-return system, the current flows from the positive (+) terminal to the component being operated. The component is earthed to the car body, which is earthed to the negative (-) terminal of the battery.