To begin with a definition: the boundary between red and infrared lies at about 700 nm wavelength. This is somewhat approximate and subjective since visual and colour perception vary, but figures of 1% visibility at 650 nm, 0.4% at 700 nm and 0.01% at 750 nm (related to 100% at 550 nm) show the rapid fall-off. It is possible to see dimly through a filter with a roll-off at 720 nm (such as the common Hoya R72) on a bright day. So infrared radiation is basically light that lies just beyond red in the spectrum.
For photographers, infrared divides in two with the part that can be recorded using ordinary digital cameras and film and focused using glass lenses extending down to about 1200 nm. That is the near-infrared and it behaves basically like visible light. Beyond this wavelength, as we move into the far-infrared, esoteric indirect photographic techniques or specialised electronic sensors must be used. Another difference between the near and far ends of the infrared lies in the source of the radiation. In the near infrared, the sources are the same ones used for everyday photography; the sun, flash guns and incandescent lamps. At the far end the radiation recorded comes from the objects themselves, since anything with a temperature above absolute zero will glow in the far infrared. When you feel the warmth of a radiator as you pass close by, that is far infrared radiation being registered by your skin. (There is a middle ground which, while not recordable with a conventional camera, is not produced by the radiant heat of a body so my near/far distinction is a simplification, but I trust it will suffice.) Of course, once something becomes hot enough, red hot, that thermal radiation becomes visible.
I have said that near infrared behaves like visible light, but there are differences. Infrared photographs are dream-like and other-worldly. Live, healthy foliage both transmits and reflects a lot of near infrared; so much so that you can use this feature to test remotely for the health of foliage. Infrared light also cuts through atmospheric haze. This is one reason why aerial and satellite imagery includes infrared. In infrared light, foliage looks 'white'. Since an open sky has no infrared, the sky goes black and the clouds really stand out. For the same reason there is little infrared in shadows (which are lit by ambient light ... the sky and reflections ... rather than by the light source) and so the contrast in a scene will increase. You can get this effect to some extent by using just the red channel of a photograph (or a deep red filter with film) but the effect is greater when you move to infrared.
The white look of foliage is because the chlorophyll in the leaves is transparent to very deep red and near infrared, and that either travels straight through or is reflected back by the plant cells. This effect is much the same as the way snow reflects light, hence the snowy look of infrared photographs. It has nothing to do with fluorescence. This snowy look is called the Wood Effect, named after the man who took the first infrared landscape photos (see our history page).
Early photography, using processes based around silver salts, was unable to register red, let alone infrared. It took some time, around 100 years, before a photography enthusiast could buy a photographic plate (as it was then) which could register infrared light. This was achieved by adding dyes to the emulsion, which accepted the longer wavelength energy. By the 1950s, there was one film that dominated infrared photography and that was Kodak's High Speed Infrared film (HIE) which was available in a variety of formats including plate sizes and 35mm. It was even available as 16mm movie film. There was a quirk to the way this film was constructed which resulted in what was seen as an infrared look to its images, even though it wasn't infrared that directly caused it. Usually film has an anti-halation layer, to limit light bouncing back from the base to re-expose the emulsion, and a grey base, which also limited light being piped through the film, like an optical fibre. This latter meant that the 35mm HIE film had to be handled in complete darkness, otherwise light leaked in through the film tail sticking out of the canister. The lack of an anti-halation layer resulted in a distinctive glow around bright parts of the image: the HIE look. Kodak's HIE was not the only black and white infrared film. Konica produced one (which I used a little) as well as a Rollei branded film, which is still available, to mention just two. Since the Wood Effect actually happens at the boundary between red and infrared, films designed with an extended red sensitivity will show it too, with appropriate filtering. You can see examples of the Kodak and Konica film in my black and white infrared film gallery.
Kodak also produced a colour infrared slide film, Ektachrome Infrared (EIR). This utilised colour-channel swapping (Infrared into red, red into green and green into blue) to give a defined false colour palette. This has also been known as Aerochrome film. Since healthy foliage reflects a lot of infrared, it shows as bright red. The colour shifts towards magenta if the foliage is 'stressed' so this can be used to detect problems in both the foliage itself and the environment, such as revealing buried structures that impede the plant roots. You can see examples shot with the two most recent versions of EIR in my colour infrared film gallery.
Digital camera sensors are as sensitive to infrared wavelengths as they are to green, so here the difficulty is filtering out infrared contamination of ordinary colour photography. This means that an out-of-the-box digital camera has an infrared-blocking filter built in. To take an infrared photograph through a suitable filter you must either take a very long exposure or remove the existing infrared block. Many photographers get their camera modified to either remove built-in IR filtration altogether, and choose a suitable filter in front of the lens, or replace the block with an infrared-pass filter of some kind.
Even with visible light filtered out, colour digital cameras will show colours unless the infrared-pass filter only passes deeper infrared such as beyond 800nm. These colours are artefacts of the camera sensor's Beyer filtering and do not represent anything 'real'. Nevertheless, interesting results are possible. Black and white images can be derived from whichever combination of channels gives the preferred result and you can see some examples in my black and white digital gallery. The colour images as taken by the camera will usually show an orangish sky but the colour balance can be made more pleasing by swapping red and blue channels, or inverting the colour components in L*A*B colour space. The colour balance is then usually adjusted to make the bright foliage look neutral, white or light grey.
If the only filter used on an otherwise unfiltered sensor is yellow then it is possible to emulate EIR. A yellow filter does not pass any blue (but does pass infrared, red and green), so the blue channel should only record infrared light. The red and green channels will record their correct colour plus infrared, so by subtracting an appropriate amount of the blue channel from the red and green channels you should end up with infrared, red and green information. There is one shot, of the Millau Viaduct, taken using this method in my colour digital infrared gallery.