With Photoshop, the more you know, the more you realize you don't know. I think that's one of the qualities I like best about Photoshop - there are continually new frontiers to explore.
In this lesson, you'll get an introduction to the Blend If controls. This is a simple and powerful tool for those of you who use layers in Photoshop. (For those of you who do not use layers in Photoshop, I strongly recomment you add that to your "To Learn" list).
To get the most out of this lesson, have a look at the video, then open a photo and play around with what you've learned. If you get stuck, have another look at the video.
Once you have the hang of it, you'll come up with dozens of ways to put this tool to creative use. Try building some of your ideas in to Actions, for even more productivity gains. That's where the fun really begins. Best of luck!