The focal length of your lens complicates this whole flash thing. If you're using a very short focal length then you have a very wide field of view, with a longer focal length you have a narrower field of view, this is basic lens stuff. With a narrower field of view though you don't need light from the flash covering as wide an area, whats more, you're probably shooting a subject that's farther away so you need a longer reach with your flash. Conversely, when you're shooting wide angle, you need the flash to cover a very wide area. If your flash coverage isn't wide enough to cover the whole area, then you'll have a vignetting type darkness around the edges of your frame.

These days most flashes have a built in zoom facility. Optical elements within the flash head can zoom in or out to focus the flash's output to a wider or narrower area. When I turned the flash on just now, you probably saw something inside the flash moving around. If you're using a flash unit made by your camera vendor, or one that's very compatible with your camera, then it's likely that the camera will automatically communicate focal length information to the flash and zoom the flash for you. As you zoom the lens you'll hear the zoom on the flash changing and you can most likely see a change in the flash head, here's what I'm talking about.

As I zoom the flash I gotta wake the flash up here half press the shutter button there as I zoom the lens. You can hear the little motors inside working. And if your watch in here you can see it changing. You can also overide this automatic zooming though using controls on the back of the flash. If you want to create more of a spotlight effect, you can zoom the flash in manually independently of the focal length of the lens. This gives you an option to create more stylized effects while you're shooting. Most of the time though, I just let the camera handle the zoom on the flash.