Many of us do not think twice about the usage of our digital camera's flash assuming that the camera will use the flash automatically as needed. Flash however can and should be manually set in many scenarios resulting in high quality digital photos.
The most common usage of flash photography is when there is not enough ambient light for example when taking an indoor digital photo in a dark room. There are many other ways in which you can use your digital camera's flash to get high quality digital photos. One such usage is fill-in flash.
Before setting your flash to manual mode and making more efficient usage of it you should know a bit more of the technology and history behind flash photography. Flash photography has been around for more than a century. During the early days of photography flash was implemented as a powder that was literally lit by either fire or electrical current. At that time flash photography was a risky business. Digital cameras today use a safe implementation by utilizing electronic flash tubes that are automatically synchronized with the camera's shutter.
You have two options for using flash with your digital camera. The first option is using the digital camera internal flash. Practically all digital cameras have build-in flash units. Most cameras also allow the usage of an external flash unit. Such flash units can be either mechanically attached to the digital camera or they can be connected to the camera via a cable and mechanically positioned on a tripod or any other mechanism. They are synchronized and controlled by the digital camera. External flash units vary in price and features. They can have different maximum light energy that they can emit and different mechanical capabilities (tilting, skewing).
In automatic flash mode the camera sensors evaluate the amount of ambient light in the scene. The digital camera fires the flash if the amount of ambient light is not high enough. There are limitations to the cameras automatic sensors resulting in either firing the flash when it was not needed or vice versa.
In some scenarios the usage of flash can result in poor digital photos. For example when the object is too close to the digital camera the flash light will be too strong and will wash out the object. Another example is in scenarios where the flash creates unwanted shadows in the digital photo. Yet another example is exaggeration of details such as when shooting a digital photo of an older person the skin wrinkles and imperfections details can be overly detailed.
Digital camera's flash units have a certain effective range. This is a limitation of how much light energy the flash unit can emit. Internal flash units usually have shorter range than external flash units. If the object in the photo is outside of the flash range - the flash will not be effective and the object will be dark. On the other hand if the object is too close to the flash unit or the flash unit emits too much energy the object will be washed out. If your object is outside of your flash unit effective range you should turn off the flash and use slow shutter photography preferably with a tripod or another stabilizing mechanism. If your flash units allows the setting of the light energy that will be fired (usually by setting the distance to the object) - make sure that it is set right to prevent washed out objects.
In some scenarios there will be enough ambient light to take a digital photo but without the usage of the flash the digital photo quality will be very poor. In such scenario if the camera is left on automatic flash mode it will not fire the flash. For example daytime photography with an object that is shadowed. If the object is wearing a hat it can create shades on the object's face or when the object is lit from the side the object's nose can create shades too. Putting the flash in manual fill-in mode will force the flash to fire. The flash will lit those shadowed areas and prevent the shades in the final digital photo. The object of course must be in effective flash range. Another example is an object that is lit from behind such as when taking a digital photo of an object against a sunset. Without a fill-in flash the photo will likely be just a dark silhouette of the object.
These were some basic concepts behind flash usage. There are many other advanced options for your digital camera flash. For example bounce flash can result in great digital photos - in that mode instead of pointing the flash directly at the object it is pointed to some reflecting surface like a wall or a special reflector. The result is more natural light and color rich digital photos.