Layer groups in Gimp 2.8 have enhanced how layer modes work. I wrote an eBook that goes into more detail on layer modes, as well as how layer groups can help enhance your editing further. Get it here: http://amzn.to/17bcto8
This Gimp Video Tutorial talks about the very basics on how layer modes work.
For a list of the tutorials mentioned in the video, visit http://gimpedtutorials.com/index.php/how-to-use-gimp-for-beginners-lesson-6-layer-modes/