NOTE: THIS APPLIES TO THE UK ONLY. IT MAY APPLY TO OTHER COUNTRIES, BUT THAT'S FOR YOU TO FIND OUT!
If you ever come to replace or fit a light fitting in a house, you may notice a ridiculous number of wires (up to 12) meeting at the fitting (e.g. ceiling rose). This is because of the ring main system common in domestic electrics for many years now. This node explains what all those wires are.
The principle behind the ring main is that all light fittings on a circuit are linked in a ring topology. So each light fitting has a 3-way cable (neutral, live and earth in, and another one out), before you even start connecting the light and the switch.
Light fittings usually have four connection points - one for each of the three ring main lines (E, N and L), and one for "switched live". Connect the input to the switch to ring live, and the return from the switch to switched live. Connect the light across switched live and neutral.
So we have:
- Live: ring main live in and out, switch input (3)
- Neutral: ring main neutral in and out, light return (3)
- Earth: ring main earths and any switch and light earths (2+)
- Switched live: switch return, light input (2)
If you are unsure about any of this, consult a qualified electrician.