According to Your Dictionary¹:
- Abbreviations: St. for Street and Saint, Mr. for mister
- Acronyms: radar for radio detection and ranging
- Initialisms: aka for also known as, FBI for Federal Bureau of Investigation
The difference between acronymns and initialisms is that acromyms form new words, while initialisms are pronounces letter-by-letter.
People who are familiar with programming, databases, or library science are well familiar with the ID abbreviation, which stands for identifier.
But there is another one "thing", UID, which stands for unique identifier. It doesn't look as a pure abbreviation (because there are two words) or a pure initialism (because the first words is trimmed to two letters). So what it is?