If you can afford it napp has a seminar for photographers and photoshop by Ben Willmore and 80% of the class was curves, not only how they work but where you can use them to fix your images etc... I had a basic understanding of curves before but his seminar really opened my eyes...the best $80 i've ever spend on photoshop education.
If thats not an option you might find some books on curves or internet tutorials. Heres a tutorial that focus's on color correcting. http://www.roguepaddler.com/curves.htm