CDT files are so-called CorelDRAW Image Templates, so picture templates for CorelDraw. CorelDraw is a vector graphics program that is part of CorelDraw GraphicsSuite.
CDT files contain information about properties for new images, such as fixed graphics and objects, fill and gradient settings, and document settings such as dimensions and rasters.
It should be noted that template files from newer versions of the program can not usually be imported into older versions and versions X4 and X5 are only backward compatible with files up to version 7. InkScape supports versions V7 to X3.