A histogram is a simple bar graph that shows the range of brightness levels that make up an image and the prevalence of each of these shades.

The Levels command lets you adjust luminance levels on a channel-by-channel basis. The upshot is that you can increase contrast, correct for color cast, and make a bad image good. The Curves command gives you independent control over highlights, shadows, midtones, quartertones, three-quartertones, and much more.

Hue/Saturation continues to do something that no other feature does: It lets you edit one range of colors independently of all others–without defining a selection or mask–all from inside a single dialog box or palette.

Both raw files and negatives contain deep information that will ultimately yield superior images. But where negatives require you to dabble in precarious and sometimes toxic chemicals, raw images have been known to submit to the slightest of numerical adjustments without the slightest provocation.

HDR stands for High Dynamic Range.  To create an HDR photo you must shoot lots of photos and merge them together.  You can shoot one properly exposed, one over exposed and one under exposed and blend them into a single deposit.  To fake and HDR photo you can take a photo into photoshop and manipulate it. By using lab color, shadow/highlight, curves, and high pass.