Using the built-in flash on your camera is a reliable, quick fix for situations that demand more light than what is available in the ambiance. However, this flash has several limitations that can be circumvented with the use of a mounted flash. A mounted flash is a separate, larger flash that can be attached to the hot shoe of your dSLR (sorry, compact camera users, but this accessory is for dSLR users only).
The most common problem with the use of a built-in flash is its lack of projection. If you try to use this flash beyond a distance of twelve feet, you will start to see that its reach begins to diminish significantly. A mounted flash, however, will give you much more power so that you won't have to remain so close to your subject. Using a mounted flash also has the benefit of allowing you to control the intensity of the flash so if you should decide to move closer to your subject your pictures won't be completely bleached out by the power of the flash.
When using a mounted flash you will need to be mindful of the shutter speed that you're using. Most cameras require that you use a shutter speed of 1/125^th or slower because anything faster than that will not be enough time for the flash's light to reflect off of your subject and return to the camera, which will result in horizontal blocks of underexposed areas in your photos. Luckily, through the workings of the hot shoe, most cameras will automatically sync to the appropriate shutter speed and aperture. However, you can choose to manually adjust the settings on your camera, so it's really the photographer's prerogative in this situation. In general, most cameras are smart enough to sync to the optimal settings.
One of the greatest assets of using a mounted flash is that you can swivel and rotate the flash so that it will not be directly firing at your subject. When using a mounted flash indoors, adjust the flash about forty-five to sixty degrees so that it's tilted upwards towards the ceiling. When you take the photo, the flash will bounce off the ceiling first and then land on your subject before reflecting back to your camera. Doing this will cause the flash's light to appear more natural in your photo as well as illuminating some details in the background so you don't have dark black shadows behind your subject.