headphone vs earplug

headphone and earplug both are nouns.

Word	Noun	Adjective	Verb	Adverb
headphone	Yes	No	No	No
earplug	Yes	No	No	No

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

headphone: electro-acoustic transducer for converting electric signals into sounds; it is held over or inserted into the ear
earplug: an earphone that is inserted into the ear canal

Other hyponyms of headphone include receiver, telephone receiver.

headphone (noun)	earplug (noun)
electro-acoustic transducer for converting electric signals into sounds; it is held over or inserted into the ear	a plug of cotton, wax, or rubber that is fitted into the ear canal for protection against the entry of water or loud noise
	an earphone that is inserted into the ear canal