All objects emit a certain amount of infrared radiation. Generally speaking, objects that are hot by human standards emit more of it than those that are cold. This radiation can be detected by a special camera in the same way that a normal camera detects visible light. An infrared camera shows hot areas as white, and cool objects as black. It is independent of the level of ambient light, and can work in total darkness. This makes it useful for rescue operations in smoke-filled buildings and underground.
Images from infrared cameras tend to be monochromatic, because the cameras are generally designed with only a single type of sensor aimed at a particular wavelength of infrared radiation. This is simpler than a colour camera, which has three types of sensor tuned to red, green and blue light allowing it to show colour pictures, but is adequate for night-vision applications.
For use in temperature measurement, the image from an infrared camera is sometimes processed to convert it to colour. This is called false colour, and has no relationship to the true colours in the scene. Each colour is simply assigned arbitrarily to a certain range of intensities of the monochrome signal, in the same way that arbitrary colours on a relief map represent different height ranges. For example, the brightest (warmest) parts of the image might be coloured white, intermediate temperatures might be shown as reds and yellows, and the dimmest (coolest) parts as blue. A scale should be shown next to a false colour image to relate colours to temperatures.
Modern digital cameras are based on CCD chips. By the nature of the materials used in them, these chips are sensitive not only to visible light but also to infrared radiation. For example, the camera can 'see' the light of your TV remote control when pressed.
Many modern digital cameras have a filter to remove infrared light to avoid the colours being wrong.
Some other cameras have a special night mode or night vision mode, where an infrared LED illuminates the scene and the camera is in infrared (only) mode. The resulting picture is black-and-white.