How Curves Work
One thing I'm always trying to encourage new photographers to do is learn the curves tool. The curves tool provides a simple, extremely powerful way to brighten, darken, and add contrast. After reading this section, please watch the masking demonstration video above to see how curves and masking combined can quickly transform an image.
Curves are a way to alter the brightness and contrast of an image. Curves work somewhat similarly to the equalizer on a sound system (A more accurate analogy would be to a dynamics processor). However, instead of making bass sounds or treble sounds relatively louder or softer, with curves you can make the dark areas or light areas darker or lighter. In this way you can increase or decrease contrast, and control in a sense where that contrast occurs.
From left to right, the curves graph goes from dark to bright, just as the equalizer goes from bass to treble. However unlike the equalizer, the vertical component of the graph also goes from dark to light, rather than quiet to loud. This is why the base line of the curves is diagonal. The bottom left point means that "pixels which are initially black (are on the left) will remain black (on the bottom)". The top right point similarly means that "pixels which are initially white (on the right) will remain white (on the top)".
The three curves shown above represent brightening, darkening, and adding contrast. In the first picture, the curve lies entirely above the diagonal, meaning every pixel regardless of brightness will be lightened. Likewise, in the second image the curve is below the diagonal, darkening all pixels. In the third picture, the left half of the curve is below the diagonal and right half above. This means all pixels darker than 50% brightness will be darkened, and all pixels brighter than that will be brightened. This increases the contrast of the image. By changing where the curve crosses the diagonal, you can change the inflection point of the contrast effect.
It's possible for a curve to be applied in different manners. Currently Filterstorm Neue only has RGB curves (the curve you set is applied to the red, green, and blue channels separately), but this will be changed in the future to include luminance curves (the curve is applied to the brightness value of each pixel), and individual channel curves in the future.