The normal image is some wacky inverted (it isn't inverted in the classical RGB sense) image, the mouse-over image is greyscale.

The illusion is that the human brain(/eye) is fooled into believing that it knows what colours should be there, but is currently not receiving that information (much like the way that most of what we 'see' is actually mono-chromatic, but the 'colour' is added by the brain/eye).

The original images are located here:

Indeed, they're not "inverted", but swapped with their complimentary colours. Staqring at the image for a while brings the brain to compensate for the colour saturation and in so doign corrects the greyscale image into the full colour one you see in the end.

You can see the process undo itself somewhat too, if you look slightly off centre briefly and then back after you switch to greyscale you will see the colours still, but with much lower saturation as your brain readjusts to the real image.