Feature Column from the AMS
http://www.ams.org/featurecolumn/archive/bezier.html

A nice refresher for Bézier curves

If I were forced to give up all but one image adjustment tool, I would keep curves. Hands down. The curves adjustment tool is an integral part of every professional’s knowledge base and image editing package. Even if you’ve taken a good photo, chances are it can be improved or it needs to be adjusted to work in a collage or collection. Or even to just to intensify a mood. You can always make a good thing better – and curves is a one-stop-shop way to do that. With curves you are able to: Adjust the over-all contrast or tonal range, Adjust the local contrast or tonal range, Adjust the color. Let’s jump in and find out how. It’s simpler than it looks.

Nice visual guide to using that pesky curves tool!

Tutorial sobre el uso del ajuste de Curvas en photoshop

Photoshop’s Curves is a flexible control that can brighten or darken parts of a layer based on the layer’s luminosity. Editing tones in an image—not just grays and not always photos—can do more than fix highlights and shadows. Curves can be used to edit photos, masks, graphics and even hues. But using it requires a little know-how and imagination. Read on for more details about what Photoshop curves are, as well as how to use them properly for your designs.

Taking Photoshop’s Curves Beyond Highlights and Shadows | Webdesigner Depot