The Brightness-Contrast tool adjusts the brightness and contrast levels for the active layer or selection. This tool is easy to use, but relatively unsophisticated. The Levels and Curve tools allow you to make the same types of adjustments, but also give you the ability to treat bright colors differently from darker colors. Generally speaking, the Brightness-Contrast tool is great for doing a "quick and dirty" adjustment in a few seconds, but if the image is important and you want it to look as good as possible, you will want to use one of the other tools.