Headlights need no introduction – they're the beams of light that makes sure you don't crash into a hedge on a dark night! They aren't particularly complex but it's well worth knowing your way around them should you ever need to change a blown bulb.
In most headlights you're likely to find the main/dipped beam, the sidelight, and in many cases the indicator (which will be obvious as it will be orange).
Depending on the age of your car you may also have daytime running lights (DRLs) which come on whenever the engine is running, although these sometimes employ the services of the sidelight, rather than having a dedicated bulb.
Some cars also have cornering lights, which come on, as the name suggests, when the wheels turn. Sometimes the actual bulb moves, and on other applications the bulb is aimed 'outwards' rather than forwards, and responds to steering input.
It's well worth familiarising yourself with your bulbs to make sure you know what's what. There are three main types of bulb that you're likely to find in your headlight, and it's not uncommon to find different types of bulbs within the headlight depending on what the bulb is used for.
Until relatively recently all headlight main/dipped bulbs were filament bulbs. They operate in the same way as a regular household bulb in that an electrical current heats up a very thin metal filament.
In most cases the bulbs are filled with halogen gas which allows the bulb to run hotter and therefore brighter.
In some cases bulbs are filled with xenon gas (don't confuse these with xenon HID lights!) which can give a whiter, brighter light.
Indicators, and sidelights also commonly use this technology, although they are steadily being replaced with LEDs.
HID stands for High Intensity Discharge, and they don't have a filament like regular bulbs. They operate on a gas discharge principle and the light comes from an arc of electricity that jumps across two electrodes contained within a glass tube filled with xenon gas.
They require a ballast which is a small box of electronics that both starts the light and controls its output. They are much brighter and whiter than regular filament bulbs and generally last longer (around 2000 hours).
The bulbs cost considerably more than regular bulbs, but fitting isn't any more complex.
LED stands for Light Emitting Diode, and due to their low power usage and ultra long life LEDs are increasingly used within headlights.
They are only used as the main/dipped beam on very new, high end cars, but they're increasingly being used for indicators and sidelights.
It's possible to upgrade your filament bulbs to LEDs but be aware that due to their low power consumption they can trigger warning lights on your dash as the car may think the light isn't working.
If this occurs you need a 'CANbus LED' which are fitted with a tiny resistor to simulate the power usage of a normal bulb.