Acronyms are a type of abbreviation. They are formed by the first letter of each word in a phrase and usually, but not always, written in capital letters. An acronym is pronounced as a word:

ASAPAs soon as possible
HIRCHuman–industrial robot collaboration
NASDAQNational Association of Securities Dealers Automated Quotations
NATONorth Atlantic Treaty Organization
PINPersonal identification number
POTUSPresident of the United States
SARSSevere acute respiratory syndrome
SWOTStrengths, weaknesses, opportunities, threats
UNESCOUnited Nations Educational, Scientific and Cultural Organization
WADAWorld Anti-Doping Agency

Some words created as acronyms have become so common that people do not know they are acronyms. Some examples:

laserlight amplification by stimulated emission of radiation
radarradio detection and ranging
scubaself-contained underwater breathing apparatus
sonarsound navigation and ranging
taserThomas A Swift's Electric Rifle

There are other abbreviations formed by the first letter of each word, but they are pronounced as individual letters. These abbreviations are called initialisms. Some examples:

B2BBusiness-to-business
BMXBicycle motocross
CEOChief executive officer
CIACentral Intelligence Agency
DIYDo-it-yourself
FAQFrequently asked questions
FBIFederal Bureau of Investigation
IPOInitial public offering
NHLNational Hockey League
RFIDRadio frequency identification
WWWWorld Wide Web
An RFID tag.
Detail of an RFID tag used on a garment

The most common initialism is probably OK. It is such a popular abbreviation that it deserves its own blog post.

Communicating on the internet has created many abbreviations:

2F4UToo fast for you
AFKAway from keyboard
BBSBe back soon
LOLLaughing out loud
KISSKeep it simple, stupid
ROFLRolling on the floor laughing
YOLOYou only live once

How the first letter in an abbreviation is pronounced determines whether the indefinite article should be written a or an. Compare the following:

A UNESCO spokespersonAn unknown person
An FBI agentA federal agent
An HR managerA human resources manager

In my next blog post you can read about backronyms.