Fog lights are the lights mounted on the front of a car or truck which help visibility in foggy or misty conditions. They come in a variety of colors and intensities, but virtually all vehicles have them, and their assistance can be crucial to navigating poor weather.
Traditionally, fog lights are yellow, and the reason why is the subject of some speculation. A common claim is that they need to be a single color light rather than white light, to minimize dispersion as the light hits the water vapor and scatters in different directions. It is often said that though red might be a more ideal color, it already has connotations to drivers —- it is used for both stop lights and brake lights. Yellow, then, would seem to be the next-most-suitable choice, because it has the next longest wavelength of visible light.
The problem with this idea, which sounds plausible scientifically, has to do with the size of the water molecules in fog. The molecules of water vapor are large enough that dispersion does not occur in any meaningful way, making the wavelength of the light irrelevant. It is possible that the first car companies to utilize fog lights were not aware of the impact that the size of water vapor molecules would have on the dispersion, and so believed that by choosing yellow lights they were minimizing the blur the lights cast. More likely is that yellow was chosen because of its connotations in the West with caution. Yellow lights and yellow signs are used to indicate that a driver should slow, look for obstacles, generally use increased levels of caution -- precisely the activities one wants other drivers to exercise when driving in heavy fog.
Many modern fog lights are halogen lights, which allows them to burn at much hotter temperatures than traditional lights, while retaining a low burnout rate. Halogen lights tend to have a tighter beam than incandescent ones, which yields an additional benefit. A special class of fog lights seen on some newer cars are high-intensity discharge lamps. These lights may use a number of different minerals to operate, including mercury, sodium and halide. These are often distinguished by a purplish hue, rather than the more traditional yellow color.