listen vs listen in

listen and listen in both are verbs.

Word	Noun	Adjective	Verb	Adverb
listen	No	No	Yes	No
listen in	No	No	Yes	No

As verbs, listen in is a hyponym of listen; that is, listen in is a word with a more specific, narrower meaning than listen:

Other hyponyms of listen include hear out, advert, attend, give ear, hang, pay heed, hark, harken, hearken, eavesdrop.

listen (verb)	listen in (verb)
hear with intention	listen without the speaker's knowledge
listen and pay attention	listen quietly, without contributing to the conversation
pay close attention to; give heed to