By following this tutorial you will learn the concept of selections in GIMP. Whenever he says "layer mask" he really meant "selection mask".
The video was uploaded in 640x480 resolution, so when YouTube enables high-res streams you will be able to watch this video in high-res. The 320x240 version is a bit blurry, but the original 640x480 video is really sharp.
This video was created on Ubuntu  using recordMyDesktop , Cinelerra CV , GIMP , MPlayer / MEncoder , a lot of trial and error and fair amounts of general computer skills and Linux knowledge. I hope you enjoy it!
Feel free to spread this video in YouTube-embedded format as much as you like. The video is licenced under the Creative Commons Attribution-Share Alike 2.5 Sweden Licence.