Dynamic Range

        Dynamic Range in photography refers to, in a single image, the maximum and minimum of measurable light intensities. Such as the highlights of an image and the shadows of the image. A camera with a particularly low dynamic range may easily blow out the highlights and have shadows with little detail. But a camera with a higher comparable dynamic range will keep more detail in the highlights as well as the shadows. Exposure is measured in f-stops. Zero is a neutral exposure of the scene, and will typically have the most natural look. 2 stops or -2 stops are over and underexposed images. In these the highlights or shadows are exaggerated. The human eye in a general sense can see a range of 24 f-stops. But more accurately since we only look at a small amount of our world at a time, this dynamic range estimate is around only 16 f-stops. DSLR's can usually see around 9-14 f-stops (its worth noting that the camera used for the below pictures have the capability of 14 f-stops of range), and film, depending on the type of film, can usually see up to 14 f-stops. 
        Since this is an introduction to HDR (High Dynamic Range) photography, and HDR image is multiple images of varying exposures to reveal detail in highlights or shadows that would, under a single normal exposure, would be difficult or even impossible to see. 

Below is an under-exposed image of my headphones. This exposure brings out the detail in the brighter parts of the image. However the rest of the image is too dark. 

Now this is an over-exposed image of the same scene. Now the details in the darker parts of the image are more visible but the highlights are overblown.

In both of the images look at the texture of the headband and ear cups. This is where it is most easily shown what I mean about viewing detail that is either washed out or too dark. Now the next image is both of these images and the zero exposure combined in an HDR. For the sake of visuals, the final HDR has been tone corrected to look more like it did in real life. Since this isn't a particularly high contrast scene and for example, I have left out the zero exposure image because the dynamic range of the scene is really close to being within the dynamic range of my camera. But in this image most of the detail in both the shadows and highlights are now visible.