A - Brightness/Contrast 

The Brightness/Contrast adjustment lets you make simple adjustments to the tonal range of an image. Moving the brightness slider to the right increases tonal values and expands image highlights, to the left decreases values and expands shadows. The contrast slider expands or shrinks the overall range of tonal values in the image. 

B - Levels 

You use the Levels adjustment to correct the tonal range and color balance of an image by adjusting intensity levels of image shadows, midtones, and highlights. The Levels histogram is a visual guide for adjusting the image key tones. 

C - Curves 

You can use Curves or Levels to adjust the entire tonal range of an image. The Curves adjustment lets you adjust points throughout the tonal range of an image (from shadows to highlights). Levels have only three adjustments (white point, black point, gamma). You can also use Curves to make precise adjustments to individual color channels in an image. 

D - Exposure 

Adjusts the highlight end of the tonal scale with minimal effect in the extreme shadows. In the Adjustments Panel, the second row of buttons is used primarily for color balancing images. 

E - Vibrance 

Vibrance adjusts the saturation so that clipping is minimized as colors approach full saturation. This adjustment increases the saturation of less-saturated colors more than the colors that are already saturated. Vibrance also prevents skin tones from becoming over-saturated. 

F - Hue/Saturation 

Hue/Saturation lets you adjust the hue, saturation, and lightness of a specific range of colors in an image or simultaneously adjust all the colors in an image. This adjustment is especially good for fine-tuning colors in a CMYK image so that they are in the gamut of an output device. 

G - Color Balance 

The Color Balance command changes the overall mixture of colors in an image for generalized color correction. Color casts can easily be removed by moving the slider to the opposite color to neutralize the unwanted tone. The tonal change can be targeted to the lightest part of the image (highlights), the darkest tones (shadows) or the overall image (midtones). 

H - Black & White 

The Black & White adjustment lets you convert a color image to grayscale while maintaining full control over how individual colors are converted. You can also tint the grayscale by applying a color tone to the image, for example to create a sepia effect. Black & White functions like the Channel Mixer, which also converts color images to monochrome while allowing you to adjust color channel input. 

I - Photo Filter 

The Photo Filter adjustment mimics the technique of placing a colored filter in front of the camera lens to adjust the color balance and color temperature of the light transmitted through the lens and exposing the film. Photo Filter also lets you choose a color preset to apply a hue adjustment to an image. If you want to apply a custom color adjustment, the Photo Filter adjustment lets you specify a color using the Adobe Color Picker. 

J - Channel Mixer 

Using the Channel Mixer adjustment, you can create high-quality grayscale, sepia tone, or other tinted images. You can also make creative color adjustments to an image. The Channel Mixer adjustment options modify a targeted (output) color channel using a mix of the existing (source) color channels in the image. Color channels are grayscale images representing the tonal values of the color components in an image (RGB or CMYK). When you use the Channel Mixer, you are adding or subtracting grayscale data from a source channel to the targeted channel. 

K - Color Lookup 

The new Color Lookup Adjustment Layer in Photoshop CS 6 has several options (3DLUT File, Abstract, and Device Link) that are used to load different “looks”. These looks are achieved by remapping every color in the image to a different one using a lookup table (LUT). I think that many photographers and designers will find their resulting color shifts quite interesting. You can think of these tables as a sort of meta-adjustment, a way to apply pre-packaged adjustments (sometimes lots of adjustments together) in one step. 

L - Invert 

The Invert adjustment inverts the colors in an image - making a negative of the positive image or vice-versa. 

M - Posterize 

The Posterize adjustment lets you specify the number of tonal levels (or brightness values) for each channel in an image and then maps pixels to the closest matching level. For example, choosing two tonal levels in an RGB image gives six colors: two for red, two for green, and two for blue. 

N - Threshold 

The Threshold adjustment converts grayscale or color images to high-contrast, black-and-white images. You can specify a certain level as a threshold. All pixels lighter than the threshold are converted to white; all pixels darker are converted to black. 

O - Selective Color 

Selective color correction is a technique used by high-end scanners and separation programs to change the amount of process colors in each of the primary color components in an image. You can modify the amount of a process color in any primary color selectively— without affecting the other primary colors. For example, you can use selective color correction to dramatically decrease the cyan in the green component of an image while leaving the cyan in the blue component unaltered. 

P - Gradient Map 

The Gradient Map adjustment maps the equivalent grayscale range of an image to the colors of a specified gradient fill. If you specify a two-color gradient fill, for example, shadows in the image are mapped to one of the endpoint colors of the gradient fill, highlights are mapped to the other endpoint color, and midtones are mapped to the gradations in between.