Flash Heads
When it comes to physically setting up an external flash, most come with swivel and bounce heads, to allow more realistic and softer lighting effects by bouncing light from a white card or reflector above or to the side of the subject. These are things that a pop-up flash cannot do.

Zoom heads on external flashes allow for coverage adjustment when the flash is used in conjunction with a wide-angle or telephoto lens. The path of light covered by the flash will be modified to suit the angle of coverage by the lens. Ring flashes are a particular type of external flash that forms a ring around the barrel of the lens - they are best suited to macro or portrait photography where large lenses can get in the way of pop-up flashes and cause a shadow.

Bulb Power
External flashes have much more power than built-in flashes. Think of how many times it took you to learn that your built-in flash's light fell short of the intended subject and lit only the near objects in your scene.

Metering with Flash
Modern external flashes allow exposure to be metered with TTL (through-the-lens) flash metering, which allows the unit to regulate the amount of light output in auto-exposure modes. The flash acutally cuts off light when enough has been dispensed on the subject. Flash exposure compensation allows further fine-tuning whereby adjustments can be made to allow more or less light than the TTL metering suggests. Thus, today's external flashes only have a guide number for flash power comparisons. The guide number on a typical pop-up flash is fairly low - perhaps 40 - whereas the guide number on an external flash easily has a guide number upwards of 100 or 200. For comparison purposes, you need to understand that a guide number of 80 is four times (two stops) more powerful than 40.

Flash Proximity to Subjectv External flash also allows you to distance the flash light source from the lens. This becomes very important for macro photography where pop-up flashes can actually cause shadows of the lens, based on its proximity to the large barrel. It is also more likely to cause red-eye in portrait shots, and means you cannot conveniently counteract another light source since the built-in flash is not moveable. With infrared or radio slaves, motion detectors or photo cell sensors, multiple flashes can be triggered remotely in sync, no matter how far away from the camera they are.

Fill Flash
Most accessory flashes have the ability to shoot at less than 100% power, so as to enable fill-flash in appropriate situations. Built-in flashes must be manually covered with a diffuse fabric in order to be successful at providing fill-light (e.g. brightening a flower, or combining fill-flash with slow shutter speed for fast-moving wildlife). Variable power in external flashes makes fine-tuning aperture or flash proximity to subject very easily. Some external flashes have infrared sensors that sense the distance to the subject automatically, thus synchronizing the camera to suggest a correct aperture and shutter speed.