A CTG file is a catalog file that is created on Canon digital cameras. These are created when photos are taken by a Canon digital camera. It stores the information about the number of photographs/images in each folder on camera’s storage device i.e. memory card. CTG files are stored in CANONMSC folder on the memory card. These files let your computer index the files after putting in the computer.
CTG File Format - More information
CTG files are saved in canon proprietary file format. These are automatically generated and should not be edited manually. The camera deletes these files when the corresponding photographs are erased by user.