The last stop on the Color Correction Express is Curves, the most powerful—and fear-inducing—adjustment in all of Photoshop. The basic idea is that, by curving a diagonal line on a grid, you change the brightness of the pixels in your image. Instead of the three main adjustment sliders you get with Levels (shadows, highlights, and midtones), Curves give you up to 16 adjustments. But that's not as scary as it sounds. If you arrived here relatively unscathed after getting through the section on Levels (Using Levels), you already know a ton about using Curves. For example:
You can use Curves as an Adjustment layer so that it's nondestructive (whee!), which means you can also use the included layer mask to restrict the adjustment to certain areas of your image. The Curves grid shows up in the Adjustments panel, just like Levels.
A Curves adjustment uses a histogram (Using Levels) and the same 256 shades of gray you saw in Levels (see the box on Good Gray Hunting). It also has the same shadows and highlights sliders (though no midtones slider), and it harbors the same trio of eyedroppers for resetting the black, white, and midtone points (Output levels). So far so good!
You can Option-drag (Alt-drag on a PC) the shadows and highlights sliders to find the darkest and lightest areas of your image, like you learned on The Levels Eyedroppers.
You can use Curves to correct your image using the Info panel and the Color Sampler tool, and you can type target values into the Input ...