Explaining Curves

Curves is a graph that displays the tonal values in an image lying along a diagonal line. Black tones are found at the bottom left end of the line and white tones are at the top right.
The graph shows unadjusted tonal values (input values) along the horizontal axis running from black (0) to white (255). The vertical axis shows the adjusted tonal values (output values) running from black(0) to white(255).
To make an adjustment you click on the line and drag a point up to brighten it or down to darken it. This allows you to selectively adjust any tone in the image with minimal effect on the other tones.