apple vs crab apple

apple and crab apple both are nouns.

Word	Noun	Adjective	Verb	Adverb
apple	Yes	No	No	No
crab apple	Yes	No	No	No

As nouns, crab apple is a hyponym of apple; that is, crab apple is a word with a more specific, narrower meaning than apple:

apple: fruit with red or yellow or green skin and sweet to tart crisp whitish flesh
crab apple: small sour apple; suitable for preserving

Other hyponyms of apple include crabapple, dessert apple, eating apple, cooking apple.

apple (noun)	crab apple (noun)
fruit with red or yellow or green skin and sweet to tart crisp whitish flesh	small sour apple; suitable for preserving
native Eurasian tree widely cultivated in many varieties for its firm rounded edible fruits	any of numerous wild apple trees usually with small acidic fruit
	any of numerous varieties of crab apples cultivated for their small acidic (usually bright red) fruit used for preserves or as ornamentals for their blossoms