I personally find Photoshop blend modes to represent one of the secret handshakes of the Obscure Brotherhood of Photoshop. There’s no reason based on name or description that you would be able to intuitively know what these mathematical mixings of pixels will do to your image layers. But in this latest video from Deke’s new book Photoshop CS4 Channels & Masks One-on-One, Deke gives you a darn good run down on what the 25 standard blend modes actually do and how to think of them in groups for better predictive behavior. If you’ve always wanted to know what to expect from blend modes (you know, something more sophisticated than randomly clicking through them), this will help get you started.
If this whets your appetite for more, as it darn well should, the book is available now, and it contains exercises that allow you to put this revealed knowledge to use in real-world projects. Ah, Enlightenment.