Because different sources of light -- such as a candle versus sunlight -- emit different colors of light, it's important for the digital camera to adjust its color scheme to match human vision, which can interpret all white objects as white, no matter what color of light they're reflecting. Think of white balance as the base line for creating colors using a digital camera.
Digital cameras can automatically determine the proper white balance settings by finding a white object in the scene and measuring all other colors from it. Manual white balance allows the photographer to choose a white object in the scene, and the camera then adjusts other colors from that object. With preset white balance, the user can tell the camera the type of lighting conditions that the photograph will experience, such as fluorescent or sunlight, and the camera bases its white balance settings on the expected lighting conditions.