German shepherd dog vs sheepdog

German shepherd dog and sheepdog both are nouns.

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

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

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

Other hypernyms of German shepherd dog include sheep dog, shepherd dog.

German shepherd dog (noun)	sheepdog (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