Abbreviations come in several sizes:
Refers only to abbreviations that sound like—and are read like—actual words: North Atlantic Treaty Organization or NATO (pronounced NAY-toe). Some acronyms spell out real words: Acquired Immune Deficiency Syndrome or AIDS. And some acronyms become “real” words: laser began as the acronym LASER (light amplification by stimulated emission of radiation).
Refers to a series of letters that you say individually: ATM (automated teller machine), BBC (British Broadcasting Company), FBI (Federal Bureau of Investigation).
Refers to abbreviations that include the first and last letters of the full word: Ms. (miz), Mr. (mister), amt. (amount), Gov. (governor).