Spatial referencing (referred to as geo-referencing as well) is the process of aligning available EO or GIS data to a coordinate system so that further spatial analysis and image analysis tasks can be applied using these data as input.
To be able to perform spatial referencing, users have to generate the so called Ground Control Points (GCPs) with known coordinates. In case of images, the easiest features that could be used as GCPs are the intersections, isolated trees etc.
Image co-registration [aka Image-to-image registration] is the translation and rotation alignment process by which two images of like geometry and of the same geographic area are positioned coincident with respect to one another so that corresponding elements of the same ground area appear in the same place on the registered images (Jensen 2005 referencing Chen and Lee 1992).