Photo Blog Stop: Blend Modes Explained.

"In this article I’m going to give you a high-level view of what the various blend modes do, and then I’ll dig deeper into the nuts and bolts of the blend modes by explaining some of the math involved, and their interrelationships with each other. I’m not going to “show” you how the blend modes work—I’m going to “explain” how they work"

OK, so the actual title of the post is "Photoshop Blend Modes Explained"- but pretty much everything in here applies to Acorn as well (except for some of the shortcuts- I had no idea about these).