This isn't yet public on the main site, but you can access it via the direct link below:
Understanding & Using Graduated Neutral Density Filters
This applies to both physical and digital GND filters, and includes an interactive example toward the middle.
With all the buzz that HDR gets these days, I think that GND filters have too often become overlooked. Everything else being equal, a GND will produce superior results to an HDR if the lighting changes evenly in one direction (such as with most landscape photographs). Both have their place though.
As usual, please let me know if you feel anything is unclear, if you notice any typos, or just want to add something from your own experience.