greasy spoon vs restaurant

greasy spoon and restaurant both are nouns.

Word	Noun	Adjective	Verb	Adverb
greasy spoon	Yes	No	No	No
restaurant	Yes	No	No	No

As nouns, restaurant is a hypernym of greasy spoon; that is, restaurant is a word with a broader meaning than greasy spoon:

Other hypernyms of greasy spoon include eatery, eating house, eating place.

greasy spoon (noun)	restaurant (noun)
a small restaurant specializing in short-order fried foods	a building where people go to eat