Many professional photographers will advise that possibly the most important bit of equipment for a photographer (other than a good camera), is an off-camera flash. An off camera flash isn’t just for the pros; even amateurs and hobbyists looking to get those high quality, arty looking results can benefit from it.
Good photos are all about interesting lighting and interesting shadows. To create those shadows and contrasts, you need a light source other than one that is mounted on the camera. On-camera flashes will typically produce harsh light and loss of shadows in the pictures because the light source and camera are located in the same place.
Built-in or on-camera flashes may also be rather low intensity and may not give sufficient light or project the light to a sufficient distance. Another problem you can avoid with the help of off-camera flashes is the pesky red-eye effect that results from a light flashing directly into the retina of the eye.
One way to create interesting light/dark contrast is to focus the flash on a particular object. The result you get from this will depend upon how much other light (ambient) there is in the room. If the ambient light is low, your off camera flash will highlight the object and draw attention to it.
Back lighting your subject with an off camera flash will give you option of creating great silhouettes, rather like someone walking across the setting sun. You can create interesting halo effects and high contrast pictures by back lighting what you want to shoot.
Bounce flash or indirect illumination helps to create a softer, natural looking but well lit picture. Here the flash is directed towards an umbrella, a white ceiling or a white reflector which indirectly illuminates the subject of your photo. A soft box will also help diffuse the light and scatter it when to reduce harshness.
Fill (fill-in) flash is another technique that is used to get the off camera flash to work to supplement the ambient light that is already present. It is used to light up an object close the camera which would otherwise be in shade relative to the rest of the scene.
Another technique you can use with off-camera flashes is setting your subjects against a black background. Here the light is concentrated on the foreground with as little light as possible used for the background.