A .CTG file on a Canon card is a catalog/index file the camera uses to manage and display photos. It typically stores image catalog information for a group of images, so you may see one created for each set of photos.

Normally, these files are handled automatically by the camera and should be removed when the related images are deleted. In practice, you generally don’t need to worry about them.

If you want to clean up a card, the usual best practice is to back up your photos first and then format the memory card in the camera rather than manually deleting support files like .CTG.