The infrared is a kind of light which appears over red in a spectrum resulting from the use of a prism. The 'infra' part comes from the fact that energy of this kind of light (converted into one quantum, that is 'a parcel') is smaller than the energy of the visible light.
The infrared radiation is not visible, but can be felt, e.g. it is hot near a bonfire. The night-vision device used by the army is a screen sensitive to infrared radiation. But can you see it with a naked eye?
Have a look at those photos of a TV remote controller. The photo was taken during 'transmission' with a regular digital camera. The video cameras operate similarly - their sensors see the infrared.
Semiconductors with so-called small energetic gap, like germanium, are used as IR detectors. It these sensors the light absorbed causes an electron pass from its fundamental band to the conduction band, and in this way a current flows in the semiconductor. The light must bring enough energy to make such a transition. The infrared light brings little energy, in spite of the fact that the radiation from a fire makes an impression to be warm. To detect the IR light, the semiconductor used has to be characterised by a small energy gap: it is only 0.6 eV in germanium, compared to 1.2 eV in silicon.
The IR light used in TV pilots is not much useful for transmitting signals at great distances; it gets absorbed by many gases, like carbon dioxide (CO2), water vapour and so on. Due to an increase of the CO2 concentration in Earth's atmosphere, more and more IR radiation is trapped and the climate warms-up. We call it a greenhouse effect.
The spectra of IR absorption by CO2 and water vapour are somewhat complementary. In this way, on Mars, where quite a lot of CO2 is present in the atmosphere but little water, the greenhouse effect is small (3K, i.e. 3°C), on Venus, where water, CO2 and also SO2 are abundant, the greenhouse effect brings the temperature up to 450°C. On Earth, the natural greenhouse affect is modest, 33K, bringing the average temperature on our planet to some comfortable +15°C.
On the figure, the IR absorption from the ammonia molecule NH3 is shown.