This involves a special type of photographic film that can pick up the infrared rays that are normally beyond the visible spectrum. The infrared rays given off by heated objects record as a sort of glowing halo around the object, which has a very cool effect in a photograph. Objects that give heat naturally seem to be the best subjects, such as humans, or vegetation. (dont ask me what kind of heat trees give, but it does look really fabulous on film). There are several rules that must be followed when using infrared film, in order to obtain the best results.
: Load the film in COMPLETE darkness. Keep the canister in its plastic container until you do load it. Infrared film is more light sensitive than regular film, and even the light that could leak in during loading it in your camera can spoil the film.
: Re-adjust your focus. The infrared rays of a subject are slightly beyond the subject's surface, so you must throw your focus to allow for that. Most SLR cameras will have a small red arrow on the focus ring which indicates where to adjust the focus for infrared.
: Bracket your exposures. Infrared film can not be properly rated by ISO/ASA standards, so it does not have "speed". I have heard it recommended to set your camera for 100 or 50 ISO/ASA, but in either case its not certain, so the best thing is to bracket your exposures on either side of a proper reading. To ensure a good exposure somewhere, take a shot at the proper light reading, and then again a couple stops above, and another a couple stops below.
Infrared photography is a very inexact science, it can change dramatically from subject to subject, day to day, temperature to temperature, etc. generally it takes a lot of intuition and even more luck.