"Infrared" is a bit of a vague term when it comes to the physics of it. The infrared spectrum can broadly be divided into the section close to visible light (i.e. just below the threshold for human vision) and into the section generated solely by heat. Respectively, these are the "near" and "far" or "deep" infrared ranges. Most digital cameras (and most infrared films) are designed to work with the "near" infrared range. As such, they don't pick up, say, the amount of heat produced by a human, as that is much too long of a wavelength. Military-grade infrared imaging can reach the deep infrared to detect such things.
Near infrared photography results in a different look, as the amount of infrared light in a scene may vary from the visible light, and many things reflect more or less infrared than visible light. For example, foliage renders very bright (like the grass in the foreground of this image) while water and clear sky are very dark. Black clothing often turns white, because it is not black in the infrared. Skin tones are usually very bright, regardless of ethnicity. Synthetic materials often respond very differently from natural materials.
Hopefully the visual differences will become more clear with some of these samples. I'm still learning how to shoot to get the most impact out of it!