HDR is short for High Dynamic Range. It is a post-processing task of taking either one image or a series of images, combining them, and adjusting the contrast ratios to do things that are virtually impossible with a single aperture and shutter speed.
An HDR image is commonly made by taking three photos of the same scene, each at different shutter speeds. The result is a bright, medium, and dark photo, based on the amount of light that got through the lens. A software process then combines all the photos to bring details to the shadows and highlights both. This helps to achieve the same task in the final photograph that the human eye can accomplish on the scene.
And yes, you can make an HDR out of a single photo as well.
Find more tutorials at Tery Ratchliff offical website and blog.