While this is NOT a curves tutorial exactly, if you have CS4 or are considering getting it, this is a look at the Curves Adjustment Layer in the new “Adjustment Panel.”
I hope you will learn a lot from this diagram I have made for you. If you are wanting to actually learn CURVES, check out my 1 hour Online Workshop. I actually teach in CS3 since so many have not upgraded yet. But that along with this lesson, should have you on your way to enhancing photos using curves.