Wire and cable sizes are classified by the maximum amperage that they can carry safely. A complex network of wires runs through the car. To avoid confusion, each wire is colour coded (but only within the car: there is no national or international system of colour-coding).
A circuit usually includes more than one component, such as bulbs in the lighting circuits. It matters whether they are connected in series one after the other or in parallel side by side.
This type of circuit is called an earth-return system any part of it connected to the car body is said to be earthed.
The fuse box is often located in a cluster of components, as illustrated here. The box is shown with the cover off.