Using GIMP: Correcting and Touching Up Your Images - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 121

2.5.9 Setting the Contrast, Brightness
and Color Intensity - Curves
The Curves function is the most sophisticated tool for adjusting the color,
contrast, and brightness of an image. It does, however, require some effort
to learn how to use it. It is easier to work with some of the other tools, such
as Levels (tonality correction), Brightness-Contrast , and Color Balance or Hue-
Saturation .
You can access the Curves tool by choosing Colors > Curves or Tools >
Color Tools > Curves .
I'll use the example images in this section to briefly explain how this tool
works.
The Options of the Function Curves
Figure 2.48
The Curves tool when it is activated.