tagger vs pos tagger

tagger and pos tagger both are nouns.

Word	Noun	Adjective	Verb	Adverb
tagger	Yes	No	No	No
pos tagger	Yes	No	No	No

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

tagger: a computer program that attaches labels to the grammatical constituents of textual matter
pos tagger: a tagging program whose labels indicate a word's part of speech

Other hyponyms of tagger include sense tagger, part-of-speech tagger.

tagger (noun)	pos tagger (noun)
a computer program that attaches labels to the grammatical constituents of textual matter	a tagging program whose labels indicate a word's part of speech
someone who appends or joins one thing to another
someone who assigns labels to the grammatical constituents of textual matter