The darker the area the more you want to increase the ISO. Higher ISO, means more sensitivity to light.
You will get images with to some extent less blur but the higher the ISO, the more noise that will appear in the image.
By noise, I mean spots and graininess.
See here for more info and examples:
http://www.dpreview.com/learn/?/Glos...itivity_01.htm
http://www.dpreview.com/learn/?/Glos...xposure_01.htm