Transforming one image geometrically so that it overlays on another image of the same area is called image registration. It does not necessarily include ground coordinate systems.  

However, in Geo-referencing, you assign ground coordinates to the pixels of any given image. In this case, you may need a bunch of GCPs to geo-reference your image. Or, if you have an already geo-referenced image, you can use that as a reference and register (using image registration) your image to the reference image. In this case, your final result will contain ground coordinates as well. 

Thanks Shabnam Jabari