White Balance Explained
White balance is the process of adjusting colour cast so that whites appear white. For example, take a sheet of white paper and place it under an indoor light - it takes on an orange tint. Take that same paper and place it under fluorescent lights it appears much whiter, or even slightly blue. Having the incorrect white balance is responsible for making photos appear very orange when indoors, or photos taken at dawn or dusk appear blue.
Most of the time modern cameras do a pretty good job of selecting the correct white balance profile to use. The most common colour profiles and corresponding temperatures and uses are shown in the illustration below.
White Balance Modes
Auto (AWB) - A simple fail-safe for snapshots, but can vary from shot to shot depending on how the camera meters the scene.
Incandescent - Used for domestic indoor lighting and will colour match more effectively, eliminating the orange colour from indoor photos.
Fluorescent - for use when photographing under fluorescent lighting, however there are many types and this setting often has many options.
Fixing White Balance
The easiest method is to select the correct colour profile on the camera; however it may be required to do so afterwards in post processing.
Most image editing packages have a white balance feature which allows selection from a list of presets, similar to on the camera. There is also often a manual mode, allowing you to manual specify what "white" is and what "black" is. From this it can generate a colour profile curve which can then be fine tuned. It should be noted that this option is for advanced users as incorrect settings here will have disastrous results.