Answer
Most visioning systems, the unaided eye, regular cameras, and even night vision googles all rely on there being a source of light shining on whatever you wish to see, and some of that light bouncing off the object, and back into your eye.
The systems usually referred to as Night vision are usually based on light amplification, and doesn't work in absolute darkness. Even the best ones needs a tiny bit of light to work with.
(Passive) infra red works in a completely different way. All things that are warmer than absolute zero radiates some thermal energy - heat radiation, in the area of wavelengths called infrared. If you have a device sensitive enough to these wavelengths, you can then see your surroundings by the radiation from the temperature differences between different objects.
(Passive) IR does work in total darkness, as long as there are enough temperature differences in the scene. You could bring it down into a deep cellar, or a cave, and you'd still see something.
As a separate sub-set there are Active IR systems as well. These work pretty much like regular visioning systems, only with the illuminator and the viewer shifted out of the range of the unaided Eye.
Answer
infrared rays lie in that part of light spectrum that is invisible to human eye, however some highly sensitive light sensors detect infrared rays, so these rays are used in night vision cameras and binoculars. these rays are focussed at some object that has to be viewed. the infrared sensors detect the reflected rays from that object and this is how we can see in the dark using these rays.
Answer
because it enables you to see warm objects, so if, for example, somebody was lost on a hill, in the dark, you would just have to scan for a hot human shaped blob, instead of using torchlight
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