Many people first heard about HDR (High Dynamic Range imaging) when Apple added the feature to iOS. HDR uses sophisticated algorithms to overlay multiple images shot at different shutter speeds to increase the dynamic range of the camera. This helps prevent losing details in a face, for example, when your subject has a bright sky behind it.
Of course, photographers have been doing HDR with more advanced cameras for quite a few years now. Mac users have some excellent high-end programs like Photomatix and HDR EFX Pro with which to post-process their photographs.
There's a low-cost option, HDR Darkroom 3, that does a good job and is very easy to use. It's the latest version of an app that has been evolving for quite some time. At the time of this writing, it's on sale in the Mac App Store for US$14.99. The usual list price is $39.99, so be sure to check if the sale pricing in still in effect.
I gave the latest version a trial run using several landscape photos taken with my Canon 5D. The photos were shot with varying shutter speeds about two stops apart with a fixed ƒ stop. The middle exposure is normally exposed.
Next, the images are loaded into HDR Darkroom 3. There's an option to align the photos, which is helpful if you were shooting hand-held rather than with a tripod. The app can also remove ghosting generated by moving trees or water.