Printed circuits are plastic sheets on which copper tracks have been printed. Components are plugged directly into the tracks. A few modern cars have flexible printed circuits. The copper tracks are printed in ribbons of flexible plastic, which replace the whole wiring system.
The individual fuses in the box are usually numbered, so that sorting out which one may have blown is simplified by referring to the numbered list in the handbook. This should tell you which fuse protects which circuit. Where they are not numbered, take out each fuse in turn and see which component stops working.
Some ancillary components can be operated without the ignition turned on by turning the switch to the auxiliary position. A radio is usually wired through this switch, so that it can be played with the engine off.
All the electrical units of measurement are interrelated: a pressure of 1 volt causes a current of 1 amp to flow through a resistance of 1 ohm. Volts divided by ohms equal amps. For example, a light bulb with a resistance of 3 ohms, in a 12 volt system, consumes 4 amps.