This is a question that I get asked from time to time.
All objects emit infra radiation, the hotter the object, the more infrared energy it emits. It is generated by rotation and vibration of atoms and molecules. The higher the temperature, the faster the movement of the atoms and molecules, and therefore the more infrared is emitted.
All objects above absolute temperature emit energy, this energy is easily intercepted and measured with detectors operating in the correct waveband. In infrared it is possible to detect radiation from objects below -80°C, but this is generally very specialised. Standard LWIR cameras can detect and measure temperatures down to about -40°C. The energy the thermal camera detects is naturally emitted from objects, the cameras themselves are passive systems.