InfraRed : Filter Wavelengths
This is a diagram showing the spectrum of light, at the far left is Ultraviolet light which you need to be in a dark room to be able to see.
The visible spectrum is a fairly narrow band of light, between 400nm and 700nm with everything else past that termed infrared
nm stands for nanometers which is used to measure the wavelengths
In photography terms, UV isnt really picked up by the sensor, but all camera sensors are able to pick up the light at the higher end of the spectrum. For normal pictures, this isnt desirable as it adds a wierd tinge to the final image, camera manufacturers add what is called a "hotmirror" to the front of the sensor looking like a light blue piece of glass, this has the job of blocking out all the light above a certain wavelength, its possible to take infrared images with most cameras but because of the hotmirror, these images take a long exposure to capture enough of the available light to register on the sensor.