I hear a lot of photographers talk about tone curve shapes: the s-curve for contrast, for crushing blacks, etc. But really taking advantage of tone curves is less about memorizing shapes that produce specific results and more about understanding exactly what they do. This great video will get you up to speed in no time.
A tone curve is simply nothing more than a set of input/output luminance values in convenient graphical form. The horizontal axis represents luminance values currently in an image, while the vertical axis represents the values that you're remapping the inputs to. Within that simple idea, though, lies a powerful tool for changing and shaping your images in all sorts of ways. Going beyond simple brightness and contrast adjustments, tone curves can allow you to quickly and easily color tone your images, remove color casts, and more, which is why it's so important to gain an intuitive feel of them to have more complete control over your final product. The best way to learn after watching this video is to load an image into Photoshop and challenge yourself to give it different looks: crushed blacks with green in the shadows, flat lighting, etc. After a few tries, you should have a good feel for how to obtain the exact result you'd like.
[via DIY Photography]