Photoshop Basics: Awesome Free Tuts by Tutsplus

...

1. What is a Mask?
2. How to Use Clipping Masks
3. How to Make Selections
4. How to Apply Layer Styles
5. Using Blending Modes
6. Understanding Adjustment Layers
7. What Does a Histogram Tell Us?
8. (etc.)  http://psd.tutsplus.com/articles/web/new-to-photoshop-get-started-here/