It's nothing more than a small portion of the electromagnetic spectrum of energy with wavelengths just a scosh longer than visible light. Human eyes can't see it but snakes (and Voldemort) can.
However, with a special lens, some cameras can photograph in that spectrum – and what a site it is! All you have to do in order to show up in an Infrared photograph is be warm blooded! (or have some kind of body heat). You put off heat – you show up, it's that simple. And the more you put off, the "whiter" you appear in the picture; the less, the darker.
This type of photography can produce haunting photos with a sort of "dream-like" quality as you can see from the photograph of Yosemite above, or those below. For purists – and who shouldn't be these days – infrared is further broken down into two portions: near-infrared and far-infrared. Photography uses the near-infrared range and thermal imaging the far portion.
Here are examples of Geotography using the technique. If these don't give you some ideas for your own photographs, nothing will; although, for film sourcing and other reasons it's becoming more difficult all the time to use this technique (see below).
Learn A Little MoreWhat once was not that difficult has now become a real challenge to accomplish, for several reasons. First, is that the major supplier of film, Kodak, has quit making it entirely! The only remaining supplier, who manually cut stock infrared film to size and re-sold it, has announced that he is running out of stock.
Secondly, even camera manufacturers have quit making the manual focus cameras needed to obtain tight focus on this type of film – the infrared filters needed for this type of film messes up all the new-fangled auto-focus types.
There are several tutorials about how to go about taking photographs in infra-red:
Getting Started with Infrared Photography By Fredrik Silverglimth
The Luminous Landscape
Gentle Intro to Infrared Photography
Wikipedia - Infrared Photograpy