HDR Imagery (or High Dynamic Range) uses three camera exposures-- one under-exposed (which is used for the bright areas of the photo), one properly exposed (for the medium areas) and one over-exposed (for the dark areas) to pull out subtle details from a scene that you would not ordinarily see with an ordinary exposure. All three exposures are then blended together to produce sometimes spectacular results.

Here's my first attempt at HDR with the Arc de Triomphe in Paris.