ac·ro·nym
A word formed from the initial letters of a name, such as WAC for Women's Army Corps, or by combining initial letters or parts of a series of words, such as radar for radio detecting and ranging.
in·i·tial·ism
An abbreviation consisting of the first letter or letters of words in a phrase (for example, IRS for Internal Revenue Service), syllables or components of a word (TNT for trinitrotoluene), or a combination of words and syllables (ESP for extrasensory perception) and pronounced by spelling out the letters one by one rather than as a solid word.
So USA is an initialism, MOD is an abbreviation, and today we were brought to you by the letter T and number 7. You might be oosa citizens, and that's pretty bad until you think about me being an Eww! (EU) citizen.
Anyway... I might just go kill a sirian werebull now all this sam talk has whetted my appetite...