word string vs syntagm

word string and syntagm both are nouns.

Word	Noun	Adjective	Verb	Adverb
word string	Yes	No	No	No
syntagm	Yes	No	No	No

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

word string: a linear sequence of words as spoken or written
syntagm: a syntactic string of words that forms a part of some larger syntactic unit

Other hyponyms of word string include sentence, syntagma.

word string (noun)	syntagm (noun)
a linear sequence of words as spoken or written	a syntactic string of words that forms a part of some larger syntactic unit