Acronyms are abbreviations that form words from the first letter or letters in a series of words. For example, DOS is an acronym (as well as an abbreviation), while CPU is an abbreviation only as it does not form a word.
Abbreviations that are formed by using the first initials of separate words should not have any periods after the letters:
WAN, US, UK, PDF
Make an acronym or abbreviation plural by adding an s (no apostrophe), for example
The article (a or an) that an abbreviation or acronym takes depends on the way it is pronounced—an if it’s before a vowel sound, a before a consonant sound
an IBM computer
an FTP program (pronounced with a vowel sound)
a DOS command
a UN resolution (although UN starts ...