Photography is as much an art as it is a science and by keeping a few rules of thumb in mind, you can significantly improve the quality of your images. The flash is of course extremely important, though it's easy to use sub-optimally, or even the wrong way.
Video: Manny Ortiz
In this video, photographer Manny Ortiz shows the difference between having the flash on the camera, compared to off (and on a stand). The effect of having the flash off the camera is subtle, but certainly worth it, as Ortiz explains:
Look at that. You've got the beautiful rim light, the nice soft light from the speedlight and ... notice how nicer ... how much more flattering the light is on her.
And it's true — with the flash on the camera, you get a flatter image. It still looks OK, but you lose a lot of depth.
That said, not all of us have the luxury of owning a light umbrella and stand, but if you can find a way to light your subjects from different angles, without resorting to your camera's built-in flash, you're going to end up with a higher quality shot.