Because these things matter, if only to keep the barbarians beyond the gates for a little while longer:
An abbreviation is a shortened word or phrase. Like Wash. instead of Washington.
An initialism is an abbreviation whose letters are individually pronounced. FBI, for example.
An acronym is an an abbreviation that forms a word: YOLO, AWOL, GIF. (Sometimes, they become actual words – like laser, from Light Amplification by Stimulated Emission of Radiation.)
Look at it this way: Both initialisms and acronyms are types of abbreviations. It’s just that one creates a word and the other doesn’t.