Infrared photography is an opportunity to explore a new world, an unseen world. Our eyes literally cannot see IR light. Therefore it is an unseen world. It's different from what we are accustomed to seeing.
IR photography started in the early 20th century. It became useful during WWI, it
showed stark distinctions between vegetation and buildings. Which identified potential enemy targets such as camouflaged munitions factories and other key sites. They are now used for outdoor photography to bring out the colours that we cannot see.
source: http://photographylife.com/introduction-to-infrared-photography
Filters are used together with infrared sensitive film or sensors "in camera effects" can be obtained. Colour infrared film have 3 sensitive layers. They reproduce infrared as red, red as green, and green as blue. All 3 layers are sensitive to blue so they must be used with a yellow filter.
Source: http://en.wikipedia.org/wiki/Infrared_photography#Film_cameras