FBI. CIA. NASA. IBM. S.H.I.E.L.D. i.e., e.g., and N.B.
The English language contains dozens of abbreviated forms, which tend to fall into one of three broad categories as presented in the title of this entry. We have abbreviations like i.e., e.g., and N.B. (which stand, respectively, for id est, exempli gratia, and nota bene — “in other words,” “for example,” and “note well,” or “HEY! Pay attention to this!”); we have acronyms, where each letter stands for a separate word and the letters together are pronounced as if they were in fact a word (NASA and S.H.I.E.L.D.); and we have initialisms, which look identical to acronyms but are pronounced like their individual components (eff bee eye, see eye ay [as opposed to aye, which rhymes with eye], eye bee em).
For the most part, abbreviations will include periods (i.e., e.g., N.B.). I say “for the most part” because over time, many of the most common two-letter abbreviations, like AD, BC, RN, MD, and so on, have lost their periods in favor of a simple closed styling. There is no one right answer to “do I use periods or not” in cases like these. Check your style book, if you’re being paid to adhere to one. Otherwise, pick a style and be consistent within your writing. Bouncing back and forth is crazy-making for readers and editors. You don’t want crazy readers and editors. Trust me. You don’t. Also for the most part, we pronounce the letters (eye ee, ee gee, en bee) of abbreviations rather than the entire word or phrase. Notable exceptions to this, I think, are the states’ names (I see IL, but I “say” Illinois). And I know there are those of you who really do say, in your heads or aloud, “id est” and “exempli gratia” instead of “eye ee” and “ee gee.” I still love you.
Also for the most part, initialisms won’t use periods, either. FBI, CIA, NSA, IBM — no periods. I’m sure there’re some out there that do. I don’t know what they are, but I know better than to say “no such thing exists.” I’d be wrong. It’s difficult to pronounce initialisms like words, because — most of the time — there aren’t enough vowels to make it possible. How would you say “NSA” or “FBI” or “CIA” as a word? That’s why they’re initialisms. Also, you need to use the word “the” with an initialism that’s the subject or direct/indirect object of a sentence or clause. “The FBI arrived yesterday to assist with the investigation.” “The sheriff’s department called the FBI.” However, “FBI agents were on the scene.” (In that last one it’s a proper adjective.)
Acronyms normally don’t include periods, either. I included S.H.I.E.L.D. as an example to prove there are exceptions. (And as a nod to my comics-geek friends. Heh.) Marvel Comics came up with that one, and it’s always pronounced like the word “shield,” but it’s also always styled with periods. Originally, it stood for “Supreme Headquarters, International Espionage, Law-Enforcement Division.” However, in 1991 that was updated; now, it’s “Strategic Hazard Intervention Espionage Logistics Directorate.” (Thank you, Wikipedia. http://en.wikipedia.org/wiki/S.H.I.E.L.D. ) NASA is a real-world example. National Air and Space Administration is a mouthful, so someone somewhere in their infinite wisdom said “we’ll pronounce it like a word, and say ‘nasa.'” The presence of vowels makes this mostly a no-brainer.
ETA: Some acronyms become words over time, like scuba (Self Contained Underwater Breathing Apparatus) and laser (Light Amplification by Stimulated Emission of Radiation). I have my Twitter editor-friend Mededitor (@mededitor) to thank for this. I write on the fly, so some things simply don’t occur to me. Thanks, Med!
Then there’s the plethora of initialisms we use (I’ll admit it, I use them when it suits me) in texting or even in casual online posting. OMG. BRB. BBIAB. TTYL. GTG. We say the letters, or some of us say the entire phrase. Doesn’t matter. Some folks will call them acronyms.They represent a phrase, not an entity like NASA or S.H.I.E.L.D.
I’m not about to attempt an exhaustive treatment of this subject. There’re plenty of other blogs about that have already done so (I checked, I saw ’em, but I didn’t read ’em closely and I didn’t take notes). Also, check your style guide. Seriously. This kind of thing is covered by any major style guide you might choose. Pick a style and stick with it (unless, as I continue to say, you’re being paid to adhere to a specific one).
Time’s up. GTG. TTYL, folks.
(Credit for the below image goes to http://metaphorlookout.wordpress.com/2010/06/10/make-it-short-and-comprehensible/ )