There are two basic ranges of infra-red. Ten micron cameras, or "far infra-red" can see engine exhaust, or human body heat a few miles away, but longer distance views become very fuzzy because the infra-red light is absorbed by the air.
Four micron infra-red can see almost as well, and is far less absorbed by air, but requires a much more expensive sensor array, and lower-teperature cooling.
Many FLIR systems use digital image processing to improve the image quality. The FLIR sensor arrays often have inconsistent responses from pixel to pixel. To fix this, the response of each pixel is measured at a the factory, and a linear transform maps the measured brightness.
FLIRs are often used in fixed-wing aircraft, helicopters, and armored fighting vehicles. In warfare, they have two large advantages. First, the enemy cannot detect one. Second, they see heat, and see through fog.