wetland vs fenland

wetland and fenland both are nouns.

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

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

wetland: a low area where the land is saturated with water
fenland: 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, marsh, marshland, swamp, swampland.

wetland (noun)	fenland (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