If you were to distinguish the 3 terms, you might consider the following definitions:
- georeferencing: act of assigning a geographical location to a photo (steps 1 & 2 in the HoudahGeo workflow)
- geocoding: encoding geographical data within an image file (EXIF/XMP export)
- geotagging: attaching geographical data as tags to a file (done as part of Flickr! upload)
HoudahGeo has got all bases covered. It is your one-stop georeferencing, geocoding and geotagging solution for the Mac. Moreover it exports to Google Earth. This process falls out of scope for the above three terms.