listener vs observer

listener and observer both are nouns.

Word	Noun	Adjective	Verb	Adverb
listener	Yes	No	No	No
observer	Yes	No	No	No

As nouns, observer is a hypernym of listener; that is, observer is a word with a broader meaning than listener:

listener: someone who listens attentively
observer: a person who becomes aware (of things or events) through the senses

Other hypernyms of listener include beholder, perceiver, percipient.

listener (noun)	observer (noun)
someone who listens attentively	a person who becomes aware (of things or events) through the senses
	an expert who observes and comments on something