High Dynamic Range, (or HDR) is the name given to an aspect of photography where, in certain scenes, there are areas of competing light and contrast.
In a nutshell, HDR is one sort of work-around for scenes that are compromised, such as a sunset i.e. there is strong light in the sky, but the foreground, relative to the sunset, is very dark. The "compromise is this:
if you expose the shot so as not to blow-out the sky, the foreground will be a black mess, with no shadow detail;
if you expose the shot for the foreground, the sky will be completely blown-out.
The decision to make a HDR image almost certainly has to made at the scene and you'll more then likely need both a tripod and remote shutter release cable. (From a single shot, you can make a quasi-HDR image in Photoshop by selecting Image->Adjustments->HDR Toning - but it's not the same as doing it for real!).
The HDR process is computer driven and often involves the mapping together of several exposures, (achieved by *auto-bracketing more than one shot), with the user having the ability to edit certain parameters affecting the choice on the final HDR rendition.