ISO

By using flash, you are able to show your main focus and what it is exactly that you are trying to show. However you can do the same thing with ISO, only you don't need to flash and you won't get rid of the natural lighting. ISO is a figure on the camera that refers to film speed which refers to how sensitive the film is. So if you are trying to take a picture inside and its lighted a little, you may use ISO 50 because its not so sensitive. However, the darker the room gets, the more you may want to pump up to ISO so that it can capture the image better. In a really dark room with only a little bit of lighting, you may use the ISO 5000, although it may come out very grainy looking.