HDR provides a way to combine a range of exposures of the same scene into one image, adding significantly to the amount of data held per pixel (most digital images hold 8 bits of color information per pixel; an HDR image has 32). The result is an image with more “dynamic range”—in other words, the brights are brighter, the darks darker, and there’s much more variance in between.
If you want to create some eye-popping HDR images, this is a great primer. HDR is not a simple effect, it can drastically change the effect of your (even bland) photos.
Source: Popular Science