"The name infrared means below red, the Latin infra meaning "below". Red is the color of the longest wavelengths of visible light. Infrared light has a longer wavelength (and so a lower frequency) than that of red light visible to humans, hence the literal meaning of below red." (origins of the term from Wikipedia)
Some uses of infrared are for military surveillance, night vision, heating, communications, remote temperature sensing, weather forecasting, astronomy, art history to detect underpaintings, and more.
Infrared radiation can be used to remotely determine the temperature of an object. This is what is referred to as thermography. Infrared radiation is emitted by objects (living or non) based on their temperatures. Thermographic cameras detect that radiation and produce images of it, showing the variations in temperatures.
Infrared in "normal" or artistic photography is when the film or image sensor is sensitive to infrared (IR) light. In film photography, infrared was achieved by using a specific infrared film, either black and white, or color. IR wavelengths are longer than that of visable light, and the photography produced from it is often described as "surreal".
In digital infrared, a camera must be specifically outfitted for IR. Usually an "infrared filter" is used; this lets infrared (IR) light pass through to the camera, but blocks all or most of the visible light spectrum (the filter thus looks black or deep red). With most of these outfitted cameras, once its been converted to IR, you can only shoot it with IR (there's no quick switch back and forth). The digital IR converted cameras do not produce quite the same effect as infrared film, but they do produce a similar effect.
The visual effects of IR are typically in color and tint/shade/tone shifts. The digital IR converted cameras will produce an overall reddish or purplish image straight out of the camera (SOOC). When converted to B&W, the most noticed effect will be that green foliage now shows up in whitish tones.
Tips: Since the SOOC images are mostly reddish in color, and the filters cut down on the light, keep in mind is that your exposure or light will need to increase. I found that for IR portraits (like the one above), using a hard constant light instead of a balanced flash will help keep contrast in your image instead of muddying it down to all gray tones.
* IR converted digital cameras are a top seller here at KEH. They sell very quickly when we get them in stock so if you're interested in purchasing a used one, checking back often is recommended. We are also purchasing these cameras at top dollar if you're looking to sell.