To understand what infrared is we must understand how it relates to the entire Electromagnetic spectrum,  as well as gain perspective between the distinguishing differences between what Infrared Thermography is, as compared to Infrared Photography and Night Vision / Light intensifiers.   Each type of technology has a specific purpose and works well within the confines of that technology, but all to often we confuse the technology because they have the word "Infrared" in common.

The Electromagnetic Spectrum

The Infrared region is part of the Electromagnetic Spectrum that is dividing up all types of electromagnetic radiation.  This radiation is divided up rather arbitrarily into a number of regions based on their wavelengths: Gamma < 10 nanometers, Ultraviolet radiation, Visible light 0.4 to 0.7 micrometers, Infrared Radiation, Microwaves, Radio waves 

The Infrared Spectral Range

Infrared is a specific region of the electromagnetic spectrum that is just beyond the light region.  The Infrared region spans from 1 to 1000 microns.  
bulletNear Infrared 0.75 to 2 micrometers
bulletMiddle Infrared 2 to 6 micrometers
bulletFar Infrared 6 to 14 micrometers
bulletExtreme Infrared 14 to 1000 micrometers

Infrared Thermography 3 to 5 & 7 to 14 microns

The regions that Infrared Thermography primarily deals with is the Shortwave region from 3 to 5 microns and the Longwave Region from 7 to 14 microns.  The cameras that are used in Infrared Thermography simply see the heat that is emitted from the surface of the object that it is viewing.  These types of cameras are used for applications like routine Preventive / Predictive maintenance inspections on electro-mechanical equipment. Where the thermal image of the equipment, as well as temperature measurements, can forewarn a pending failure.  The cameras that are used in Infrared Thermography do not see through objects or into buildings through the walls. Infrared Thermograph is:
bulletNon Intrusive
bulletNon Invasive

Infrared Photography 0.7 to 0.9 micrometers 

Infrared photography involves the production of photographs by means of near-infrared radiation. This radiation that lies in a range roughly between 700 and 900 nanometers can be recorded on specially sensitized photographic emulsions. Infrared radiation actually comprises a much greater part of the electromagnetic spectrum, most of which cannot be recorded directly by photographic means. Elaborate electronic equipment is required to record heat waves as in Infrared Thermography.

Near-infrared radiation has the ability to penetrate aerial haze, which makes it possible for you to photograph distant terrain, a valuable factor in aerial photography. Even more important is the way in which foliage is recorded on infrared-sensitive emulsions. The internal structure of leaves reflect infrared strongly, so that they appear lighter in tone in the photograph than they do when viewed directly. The usefulness of infrared photography lies in the unique tonal differentiation that it produces. Conifers, for example, appear darker than the leaves on deciduous trees; diseased fruit and vegetable crops or those suffering climatic or nutritional stress can be detected before trouble becomes apparent visually. These features of infrared photography become especially valuable in pictures taken from the air.

Night Vision / Light Intensifiers 0.45 to 0.95 micrometers

In Night Vision systems Gen I/II, in low light situations, the foremost lens collects low levels of light that are reflected off of objects that cannot be seen with the human eye and focuses it on a image intensifier tube. Inside the image intensifier tube a photo cathode absorbs this low level of light energy and converts it into electrons. These electrons are then passed through a microchannel plate that multiplies them thousands of times and drawn toward a phosphor screen. When this highly intensified electron image strikes the phosphor screen, it causes the screen to emit a amplifies image that can be seen with the human eye. Since the phosphor screen displays the image in exactly the same pattern and degrees of intensity as the light that is collected by the objective lens, the bright nighttime image you see in the eyepiece corresponds precisely to the outside scene you are viewing.

This type of systems work great for seeing night time scenes in low light conditions but do not provide a thermal image that corresponds to the amount of heat (thermal radiation) that is emitted from the object. Because of this this type of  equipment is not used in Infrared Thermography.