From Help Wiki

Curves goes hand in hand with Levels. Although they both follow the same principle in how much contrast an image has, the Curves tool is a love/hate relationship. It can change how color looks in an image, it can desaturate an image, and all in between. Its certainly a tool that one should use if concentrating in photography, and the photographer is interested in fixing up an image so it stands out at its brightest.

To see all of what Curves can do; at Chromasia there is a handy-dandy tutorial, explaining in-depth about Curves and how to correctly use the tool.