HDR

HDR stands for High Dynamic Range. I could type a Wikipedia-style definition of what it is and how this effect is achieved but there is no point. You have to see it to understand. Basically, HDR is an effect where the highlights and darks in an image, that would normally not be captured in a single exposure, are combined with the mid-range exposure in order to show the highlights and shadows as closely as they would appear to the human eye. This is sometimes necessary since a camera cannot capture the highlights and shadows exactly as they appear to the human eye in a single exposure, even if it is perfectly exposed.
Photoshop CS3 comes with an automation which combines the multiple exposures into a single HDR. But, it doesn't do as good a job or have as much flexibility as the reigning champion of HDR software, Photomatix (hdrsoft.com).
Trey Ratcliff is a photographer whose work is mostly done in HDR. His blog, stuckincustoms.com, is one of the most visited photography blog on the internet. On his blog, one can find examples of his amazing work and a detailed tutorial on how the amazing HDR effect is achieved using photomatix.
Here is an example of one of my own HDR images and one of Trey Ratcliffs.