There are several different types of extractor fans available, Kitchen extractor fans normally incorporate a cooker hood and are used to expel cooking smells and steam outside. Bathroom extractors are used to expel large amounts of condensation which is often caused by a shower.
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A normal cooker hood would be wired up like the diagram here, there will be a switched Fused connection unit leading to either a junction box or directly to the cooker hood. The cooker hood normally comes supplied with a good length of flex which can be connected to the junction box above the ceiling. The wire and the 4" ducting pipe is hidden by the extractor chimney which leads outside, although you can get extractors with charcoal filters which simply filter the air.
Kitchen extractor fans/hoods should be wired into the kitchen ring main via a fused, switched connection unit.
Here you can see that instead of the wire being concealed in the Hood chimney it runs along side the extractor and to a fuse switched connection unit above the kitchen units.
A shower extractor fan is wired much differently as there is often a switched live wire to enable the extractor fan to run for a set period of time after the light is switched off- Install shower extractor fan