The single bend and double bend curves are most commonly used during post-processing, but these are not the only options. Curves can have a large number of set points, bends, and inflections. More on nonlinear curve adjustments and how they affect histograms is here. Nonlinear Curve Adjustments and Histograms Nonlinear Curve Adjustments and Histograms

These video tutorials are fairly comprehensive for the grayscale and combined RGB channels, covering the basics of contrast and brightness adjustments and also color channel curve adjustments.

The Color Balance Adjustment tool gives limited options when it comes to correcting images, whereas the Curves Adjustment tool allows to color correct specific areas of the image without affecting the rest of the image. [link]

Learn how to transform an ordinary photo portrait into a dark and evil look image using Adobe Photoshop.