I'm still working on putting together my article on using curves and histograms to edit photos, so in the meantime here are some good video tutorials on the subject. These tutorials are easier to understand because they show how the curve adjustments affect the image in a more dynamic fashion.
This first tutorial is fairly comprehensive for the grayscale and combined RGB channels, covering the basics of contrast and brightness adjustments while also hitting on a lot of little tips and tricks. The creator of this video also jumps into some of the things you should not be doing with curves so that you can avoid these situations.
This second video is a little more basic than the first, but it presents some of the material in a slightly different manner. If you watched the first and you don't have a good grasp on the curves dialog, watch this one and see if it helps.
And if you have already watched the first two videos and you still don't have a handle on things, this last one from our friend Donnie might help. He also gets into color channel curve adjustments, so this is helpful for the more advanced users.