You can find tone curves on various places. Understanding what the graph of Tone Curve shows will help to understand some of the image adjustments.
The Tone Curve shows the shadows/highlights relationship between
the input (Original) values (on X axis) and output (Result) values
(on Y axis).
The default curve is an upward from bottom left to top right corner. This represents linear relationship between the original and result - that means no changes.
As the curve become more vertical - the contrast of the image
As the curve changes more to vertical - the image becomes less contrast.
By changing Gamma or Contrast settings you changing the shape of the curve(s). This changes the tonal feeling of the image, reduces or enhances midtones etc..
|The Gamma >1.0 shifts the shadows toward lights|
|The Gamma < 1.0 shifts the shadows toward dark|
|Contrast changes both shadows and highlights. The curve become more vertical which makes greater differences in midtones.|
|Linear shift changes the brightness of the image. Here the image is darker because the whole range of input is mapped only in the darker part of output.|
|RGB Curves works like half contrast - making contrast highlights or dark, but not both at the same time.|
|Changing the R,G,B curves alone creates different color adjustments.|
|With Levels you can adjust all curves separate in various ways making almost any color corrections.|