From MerriamWebster:
Some say that the distinction between an acronym and an abbreviation is that an abbreviation becomes an acronym if it is pronounced as if it were a word. So NASA, WYSIWYG and CORBA are acronyms while IBM and HTTP are not. SCSI on the other hand is usually pronounced "scuzzy", so would be an acronym. I pronounce IBM as ib-bum, and HTTP as hit-tup, must I still call them abbreviations?
The confusion is apparently caused by different uses of the word "word": is every consecutive sequence of alphabetic characters a word, or does it have to be a pronouncable sequence of syllables?
For an abbreviation to be an acronym it needs to be a) an initialism (to be made up of the heads of its component words: for example the initial letters) and b) it should be treatable as a word.
SNAFU satisfies both of these criteria, and so is an acronym. 'Amt' fails to be an acronym for 'Amount' on both counts.
From MerriamWebster:
According to http://www.alt-usage-english.org/excerpts/fxacrony.html an abbreviation "formed from the initial letter or letters of each of the successive parts or major parts of a compound term" is called an "acronym" if pronouncable, an "initialism" otherwise.
Funny, according to the document "Intro A: Welcome to AUE and Guidelines for Posting":
If you insist on calling all abbreviations "acronyms," linguists will chuckle smugly and shake their heads at you. No one else will notice, and it'll keep the linguists amused.
Then again, linguists have mostly advanced (or retreated!) beyond both the strict prescriptive and strict descriptive viewpoints; at some juncture, if the usage shifts enough, the distinction will be moot. Word up!