Infra Red Photography is used to capture wavelengths of light that aren't visible to the human eye.  Colours, human skin, trees, leaves and plants, and all manner of other objects, reflect IR light in unique and interesting ways to that as seen in the colour spectrum humans can see. Best of all this can't be imitated using tools such as Photoshop !
Adobe give an excellent description here (see Visible Spectrum image below) :  INFRA RED PHOTOGRAPHY

"Visible light is a type of electromagnetic radiation; we can only see those wavelengths of ca. 400-700 nanometres (nm).        X-rays and radio waves operate below and above those respective limits. Infrared wavelengths range from around 700nm to 900nm". 
 

More generally, IR photography lends an ethereal, otherworldly, atmosphere to images as it 'sees colours differently' with tree leaves & plants especially (depending on the wavelength photographed in and the conversion technique) pink, yellow or white. Note these are not 'fake' colours, they are just the colours of objects as viewed in the infrared wavelengths !