Infrared photography, uses a filter that blocks most of the visible light spectrum and only allows infrared (IR) light to pass through to the sensor. These filters allow for some interesting effects. People either love it are hate it. These filters can also enhance black and white photography.
The key in infrared photography lies in the foliage (green trees, grass, etc) because the green foliage strongly reflects infrared and makes the foliage appear very bright, or white.
Digital camera sensors are usually sensitive to infrared thus interfering with normal photography. Digital cameras converted to IR are adjusted to prevent confusing the autofocus algorithm. Blockers are used to improve image quality. Normally infrared cameras require longer exposure times, although I have not experienced that so far.

I had a used Canon Rebel 2ti converted by Spenser's. For a more in-depth discussion on infrared and how it works check out their web site at http://spencerscamera.com/