IR filters (eg R72) are opaque to visible light but transparent to IR light of certain wavelengths. Used in conjunction with IR sensitive film or digicam, it can used to create images using IR light reflected off objects (just as visible is used in normal photography). IR images are often very different from the normal images that you see because the reflectance of objects to IR light can be quite different from that to visible light. So you often get images with ethereal quality - light foliage, dark sky, soft focussing effect (due to the fact that IR focusses at a diff point than visible light) etc. Can search google for lots of sites devoted to IT photography.
