The Basics of Clipping Paths and Opacity Masks - Vectortuts+
http://vector.tutsplus.com/tutorials/tools-tips/the-basics-of-clipping-paths-and-opacity-masks/
tuto bonitinho e polvo no illustrator :)
In this tutorial, we're going to cover the benefits, some uses, and the 'how-to' of two powerful methods to hide or show areas of an Illustrator document
Le basi sui tracciati di ritaglio e le maschere di opacitÃ