canker sore vs ulcer

canker sore and ulcer both are nouns.

Word	Noun	Adjective	Verb	Adverb
canker sore	Yes	No	No	No
ulcer	Yes	No	No	No

As nouns, ulcer is a hypernym of canker sore; that is, ulcer is a word with a broader meaning than canker sore:

canker sore: an ulceration (especially of the lips or lining of the mouth)
ulcer: a circumscribed inflammatory and often suppurating lesion on the skin or an internal mucous surface resulting in necrosis of tissue

Other hypernyms of canker sore include ulceration.

canker sore (noun)	ulcer (noun)
an ulceration (especially of the lips or lining of the mouth)	a circumscribed inflammatory and often suppurating lesion on the skin or an internal mucous surface resulting in necrosis of tissue