collie vs sheep dog

collie and sheep dog both are nouns.

Word	Noun	Adjective	Verb	Adverb
collie	Yes	No	No	No
sheep dog	Yes	No	No	No

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

collie: a silky-coated sheepdog with a long ruff and long narrow head developed in Scotland
sheep dog: any of various usually long-haired breeds of dog reared to herd and guard sheep

Other hypernyms of collie include sheepdog, shepherd dog.

collie (noun)	sheep dog (noun)
a silky-coated sheepdog with a long ruff and long narrow head developed in Scotland	any of various usually long-haired breeds of dog reared to herd and guard sheep