An Explanation of Photoshop Blend Modes
http://www.myinkblog.com/2009/07/14/an-explanation-of-photoshop-blend-modes/
great explanations plus photos to show you or download and "play along"
An Explanation of Photoshop Blend ModesAdobe Photoshop Tutorials | High Dynamic Range | Adobe Camera Raw | Layers Magazine
Exposure Blending in Photoshop