HDR is an acronym for High Dynamic Range. It is a photographic technique usually involving multiple exposures of the same scene shot in rapid sequence at different exposure values in order to capture a scene of bright and dark areas that your eyes are usually capable of seeing, but your camera cannot capture in a single exposure. The photographs are then merged using a computer program. In the above photo a single shot from most any camera looking towards the setting sun would show a completely over-exposed (white) sky and/or a completely under-exposed (black) foreground. Because of the local contrast, clarity and color saturation this process can work well for motorcycles.
If you shoot motorcycle HDRs, share them here! If you want to know more about HDR photography, there are stunning examples and a free tutorial here: Stuck In Customs
This is what some people think of when they think HDR (and why many hate HDRs).
But they do not always need to be so obvious.
HDRs can make even cloudy days look interesting.
BTW, If you HATE HDRs, you're in the WRONG THREAD!