wetland vs marshland

wetland and marshland both are nouns.

Word	Noun	Adjective	Verb	Adverb
wetland	Yes	No	No	No
marshland	Yes	No	No	No

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

wetland: a low area where the land is saturated with water
marshland: low-lying wet land with grassy vegetation; usually is a transition zone between land and water

Other hyponyms of wetland include bog, peat bog, fen, fenland, marsh, swamp, swampland.

wetland (noun)	marshland (noun)
a low area where the land is saturated with water	low-lying wet land with grassy vegetation; usually is a transition zone between land and water