Artwork created using the simple style of "layering". In this case the author used 79 layers.
In Adobe's site I found a tutorial about Master advanced adjustment layer techniquesIf you’ve discovered the versatility of adjustment layers in Adobe® Photoshop® CS2, and have mastered the basics, it’s time to see just what adjustment layers can do for you. In this tutorial, you’ll learn advanced adjustment layer techniques. Continue to tutorial page.