X is a DirectX model image file used by DirectX technology which is for 3D graphics rendering in games. The file format specifies 3D object structures for meshes, textures, animations, and objects.
FBX, FilmBox, is a popular 3D file format that was originally developed by Kaydara for MotionBuilder. It was acquired by Autodesk Inc in 2006 and is now one of the main 3D exchange formats as used by many 3D tools. FBX is available in both binary and ASCII file format. The format was established to provided interoperability between digital content creation applications.