Normal, consumer cameras are designed to capture what is called the "visible spectrum of light"---the light that human eyes can see. Like our eyes, these cameras capture the light that bounces off of objects and records it on either film or a digital sensor. Infrared cameras operate in this same way, but are designed to capture the infrared spectrum of light, which the human eye cannot see.
Images taken with infrared cameras show light in a unique way.
History
Infrared photography did not appear until the 20th century, primarily due to the technological limitations of the manufactured film, and its inability to capture infrared radiation. After the invention of a film that could "see" infrared light, the technology rapidly improved, including its use during World War I for nighttime aerial shots. Infrared film became a novelty for photographers, and allowed for a variety of effects that could not be reproduced with standard film.
Significance
Infrared cameras allowed photographers to capture images when other conventional tools failed. Because these cameras require only infrared light, they can be used in what the human eye would perceive as complete darkness. The addition of special infrared lights and flashes have expanded this realm, allowing for brightly exposed shots with absolutely no visual detection. This has proven a valuable asset for military forces conducting reconnaissance at night.
Function
An infrared camera can be used to create thermographic images---photographs that capture the heat that radiates from the subjects---if it is specially configured to see only the range of wavelengths that correspond to thermographic radiation. These types of cameras create the "heat vision" images many people are familiar with in which humans and other warm-blooded animals appear as bright pink blobs. An infrared camera can also be designed to see a wavelength between this spectrum and the visible light spectrum, and can capture "night vision" images instead of heat.
Types
Infrared cameras can come in film---both black and white and color--- and digital varieties. Film cameras need only special infrared film, but otherwise typically need no modifications to the camera itself. Many photographers will make use of a filter that blocks the visible spectrum of light but allows infrared through in order to prevent the light from interfering. Digital cameras are often equipped with an infrared filter built into the camera's body, as digital sensors are sensitive to both visible and infrared light. This filter must be removed to take infrared pictures, but no other special equipment is necessary.
Features
The odd shifting of colors common in color infrared photography is caused by the choice of dyes necessary to capture infrared light. These films will display infrared light as the color red, red light as green, and green light as blue. Several manufacturers include "night vision" modes with their digital cameras that involve simply moving the infrared filter out of the way before an exposure is made. These manufacturers also include limitations with this mode---such as a long shutter speed---to prevent the camera being used for nefarious purposes (some clothing is actually invisible in the infrared light spectrum).
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