This section is for those who have never used a Curves adjustment before. It will give you a grounding in the theory behind tonal curves and also how to use them. If you are already familiar with the basics of the Curves adjustment or have used tonal curve controls in other software before, feel free to skip past to the next section.
To understand how the Curves adjustment works, you must first understand how a histogram works. To do that, you must learn how to look at your image in terms of its tonality or brightness level. If you break down your image into the shadow areas, the midtones, and the highlights, you'll begin to see how the software sees your image. Now imagine that there are tones in between these tones. In fact, consider that every value of brightness from the darkest to the lightest parts of your image gets measured. If you were to plot the number of pixels in your image at each brightness level on a graph, you get something that looks like the following diagram...