Acronyms

Yeah... I thought about IPA and IPA. Those two are acronyms (from Greek ακρος, meaning "topmost, extreme" and ονομα, meaning "name"). But... isn't it an initialism?
An abbreviation (from Latin brevis, "short") is a shortened form of a word or phrase.
An initialism is an abbreviation form from initials. This is, if you see "P.O.D.", you'll read it "p-o-d", pronouncing each letter name. When you find an acronym, which is also an abbreviation from initials, you'll read it as if it were a word on its own, as in "FIFA" or "FIDE" or "laser" (this one... is not even written in capitals! It stands for for light amplification by stimulated emission of radiation).
If you are in doubt about an acronym, you could try to search it at the Internet Acronym Server or the Acronym Finder.
There is also another category: portmanteaux (portmanteau, in singular). This is a word or morpheme that fuses two or more words or parts of words to give a combined meaning. You might be familiar with the word "podcast". This word is a blend of the trademark word "iPod" and "broadcast". By the way, portmanteaux are called blends too. Nowadays, this term is considered more approppriate.