Every type of light imparts a color cast. Be it the incandescent light in your living room or the fluorescent bulb in the drawing room or even the Sun. This color cast becomes a bit of an issue when you look at them on the computer screen or make a print. Your images are either too orange or too blue.
White balance is the name of an adjustment process wherein an image is treated to display the color white as it should be, sans the color cast.
In the process, the rest of the colors are also corrected for any color cast. The process is sometimes also referred to as gray balance. This is because the process involves displaying neutral colors such as white and 18% gray as they are – neutral. 18% gray has an average reflectance and photographers routinely use an 18% gray card as a reference when shooting in changing light. This allows faster correction of the color cast when post-processing.