German shepherd vs shepherd dog

German shepherd and shepherd dog both are nouns.

Word	Noun	Adjective	Verb	Adverb
German shepherd	Yes	No	No	No
shepherd dog	Yes	No	No	No

As nouns, shepherd dog is a hypernym of German shepherd; that is, shepherd dog is a word with a broader meaning than German shepherd:

German shepherd: breed of large shepherd dogs used in police work and as a guide for the blind
shepherd dog: any of various usually long-haired breeds of dog reared to herd and guard sheep

Other hypernyms of German shepherd include sheep dog, sheepdog.

German shepherd (noun)	shepherd dog (noun)
breed of large shepherd dogs used in police work and as a guide for the blind	any of various usually long-haired breeds of dog reared to herd and guard sheep