Curves tool

Curves is a function that can be found in practically all edition tools (such as Photoshop and Gimp). It provides a flexibility that you cannot achieve using Levels.  Basically the curve defines the transformation of intensity between the inputs (what we have) in the horizontal axis and the output in the vertical one (what we want to have, the result of the transformation).

This tutorial from Cambridgeincolour.com explains this tool very clearly...

http://www.cambridgeincolour.com/tutorials/photoshop-curves.htm

I took this photo from outside of a shop (left photo), you can see the reflections on the glass.
The photo is improved significantly using Curves tool (right photo)