An ABBREVIATION is a shortened form of a written word or phrase used in place of the whole word. Some authorities consider acronyms and initialisms to be abbreviations.
An ACRONYM is a word (such as radar or snafu or NASDAQ) formed from the initial letter or first few letters of a word or a series of words (example: radar comes from radio detecting and ranging).
Acronyms are pronounced as complete words.
An INITIALISM is an acronym formed from initial letters (NYSE, AFL-CIO, NAACP, IRS, SEC).
Initialisms are prounounced letter by letter.
But perhaps more important than designation is this question: When do you use "a" and when do you use "an" in front of an acronyms and initialisms?
Most style guides say that if it is READ as though all the words were spelled out (NAACP, TVA), the acronym is treated as a series of letters, and the choice depends on the pronunciation of the FIRST LETTER (an HMO facility; an M.B.A. degree; an R.S.V.P.; an LA-based company).
If it's generally PRONOUNCED as though it were a word (NATO, HUD, etc.), the article is determined by the pronunciation of the WORD (a RICO hearing; a MADD chapter; a SWAT team; a FICA increase).