Curves

Curves are one of the most fundamental image adjustment tools in photo editing software, but they're not often well understood – and particularly the relationship between levels and curves adjustments.

The easiest way to think of it is that levels adjustments are used to maximise the tonal range or total contrast in the photo, from dense black to brilliant white, whereas curves are used to adjust the contrast within a specific range of tones.

An example might help. You might have a photo with maximum contrast that still looks rather flat. Typically you can make these pictures look better by increasing the contrast in the midtones, and you can do that by steepening the middle of the curve. This inevitably flattens out the highlight and shadow areas, though, so you get less contrast here than you had before. Often, this doesn't matter, but it does highlight a key point about curves –  that there's only a finite amount of contrast in the picture and that you use curves adjustments to control where in the range of tones that contrast is strongest.