White Balance
This is how the camera compensates for different types of light that may be shining on the scene. Tungsten light bulbs actually produce a fairly yellow light, whereas flourescent lights are quite green, and if you tell the camera in advance what kind of light is used, it can correct the
color temperature in the photo. Setting the white balance wrong can produce interesting effects - try a shot at dusk with WB set to "tungsten" - you'll get an extra-blue sky.