@RorrKonn
Yeah, but no handle snapping, that's just crazy (and unfortunate)!

@Musikman
Basically, just think of a mask as an 'intensity control' image, so that you can use that intensity image to control the intensity of whatever it is you want to do.
Anything really, from controlling the opacity of a brush stroke, to controlling how (and where) an effect filter is applied to an image.

While "Mask" is the term often used, you would probably do better searching for tutorials on "Alpha Channels".
Can't advise on the issues you had with GIMP, mine runs fine on Debian and Windows 7 64-Bit.