I answered this question in a facebook group few days ago... Here is the answer: (1) normally image registration in remote sensing refers to image matching when creating mosaic, (2) image co-register is the process of registering image bands together accurately to avoid what is known as "ghost boundary" which may be the result of band offset, (3) finally orthrectification is the process of terrain correction (relief displacement) in an image, and this requires accurate DEM and several ground control points (GCP) between the image and DEM to create the orthorectified image.
