White balance is a very useful and creative control, but unfortunately it's also very underused and often misunderstood. Essentially what the control does is to match the sensor's color response to the existing light. Most cameras have numerous white balance options that include choices like sunlight, shade, overcast days, fluorescent lighting and flash. By selecting the option that matches the type of lighting you're using, theoretically you'll get the best color match.
But using white balance is one of those confusing situations when choosing the "correct" option might not be the best choice. For example, if you're outside photographing a scene like this barn and it's a bright, sunny day, you'd think the best option would be to choose "sunny day" or "clear daylight." And that is exactly the setting that I used for the top photo here. The problem is that the photograph is too neutral. It's bland. By selecting the "shade" option (or you can use "cloudy day") instead, the camera adds extra warming filters to the scene (because it thinks it needs you want to compensate for the extra blue coloring typical of a cloudy day) and your image will turn out much warmer and more inviting (as the bottom shot demonstrates). Manipulating white balance in this way will help you get exactly the color balance that you want--whether it's correct or not.
In fact, I use the "cloudy day" (which adds a lot of warming to scenes) almost all the time for daylight scenes simply because I like the warmer look. If you want to find out what each different setting does, it's easy: mount your camera on a tripod and shoot several exposures of a scene using each of the different white balance options. That's what I did for this test, though the entire test consisted of about eight different frames and settings.