An acronym is an abbreviation of initial letters formed from other words and pronounce as one word.
While, an acrostic on the other hand is a poem, word puzzle, or other composition in which certain letters in each line form a word or words.
For the poem definition it's a poem in which the first letter of each line spells out a word, message or the alphabet.
So, it's "an acronym forms a word, while an acrostic forms a sentence"