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.
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.
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.
The fuses are usually grouped in a box or on a panel with a cover. Ideally, the box should be fixed in an accessible place — such as the bulkhead under the bonnet.