Headlights need no introduction – they're the beams of light that makes sure you don't crash into a tree 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 headlight lens housings you're likely to find the high/low beam headlight, the parking light, the cornering light (if equipped), and in many cases the turn signal or blinker (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 parking lights or headlights, rather than having a dedicated bulb.
Some cars also have cornering lights, which come on, as the name suggests, when turning a corner. Sometimes the actual bulb moves, and on other applications the bulb is always aimed 'outwards' rather than forwards, and responds to steering input or the turn signal being activated.
It's well worth familiarizing 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.
Halogen/Filament Bulbs
Until relatively recently all high and low beam headlights 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. In older models, and many fleet trucks, the bulb is sealed in the headlight lens and must be changed as a unit, which is why they are called “sealed beams”.
Turn signals, parking lights, taillights, reverse lights, and others also commonly use this technology, although they are steadily being replaced with LEDs.
HID/Xenon and Bi-Xenon
HID stands for High Intensity Discharge, and they don't have a filament like regular bulbs. They operate on a different principle and the light comes from an arc of electricity that jumps across two electrodes contained within a glass tube filled with xenon gas.
These lights require a ballast which is a small box of electronics that both starts the light and controls its output. The ballast boosts the voltage by several thousand to make the arc, then dials it back to the hundreds of volts to maintain it. HID lights 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 replacement isn't any more complex. Often a car equipped with HID lights will still use a halogen bulb for the high beam, or passing light, or a combination of HID and filament lights.
LEDs
LED stands for Light Emitting Diode, and due to their low power usage and ultra long life LEDs are increasingly used within headlights, taillights, and indicators.
LED headlights are only used as the low/high beam on very new, high end cars, and hybrids, but they are becoming more common and they last even longer than HID bulbs. LEDs are increasingly being used for blinkers, brake lights, fog lights, and more, and may last the life of the car in these applications.
It's possible to retrofit 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. In older model cars, LED turn signal bulbs may not flash properly without added resistors, or the installation of a solid state flasher/blinker relay.