lesion vs ulcer

lesion and ulcer both are nouns.

Word	Noun	Adjective	Verb	Adverb
lesion	Yes	No	No	No
ulcer	Yes	No	No	No

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

lesion: any localized abnormal structural change in a bodily part
ulcer: a circumscribed inflammatory and often suppurating lesion on the skin or an internal mucous surface resulting in necrosis of tissue

Other hyponyms of lesion include tubercle, ulceration.

lesion (noun)	ulcer (noun)
any localized abnormal structural change in a bodily part	a circumscribed inflammatory and often suppurating lesion on the skin or an internal mucous surface resulting in necrosis of tissue
an injury to living tissue (especially an injury involving a cut or break in the skin)