The method that infrared cameras work is identical to how visible-light cameras work (well, digital cameras anyways). It's called electromotive force that is given off by the photons as they travel. A photon is a little packet of light energy that has a specific frequency and energy associated with it, infrared is along the same electromagnetic spectrum as xrays, microwaves, ultraviolet, and visible light.
When a photon strikes a special material it tells the camera where the information of those photons, frequency (color) and energy (brightness), which then makes an image of it. Materials that detect infrared energy are more expensive than visible light and are usually specialized to the point where you can't view both with the same material. I'd look at thermal imaging on wikipedia and read up on it yourself. Hopefully I gave you a few things to go verify!
