Both an acronym and an initial take the first letter of a string of words to form an abbreviation.
Two examples are USA (United States of America), and QANTAS (Queensland and Northern Territory Aerial Service). Generally, the collected letters stand for something, although occasionally a letter can be left out.
In an initialism, the letters are pronounced as letters when read aloud. For example, the term ‘USA’ is an initialism, and we pronounce each letter individually: u, s, a.
In an acronym, the letters are pronounced as a word when put together. When we say ‘QANTAS’ we pronounce it ‘quon-tuss’, we do not spell the letters out. Many acronyms have now entered our language as a proper word. ‘Laser’, for instance, is actually an acronym for ‘Light Amplification for Stimulated Emission by Radiation’.
In academic writing, you must write the words in full in the first instance, and then placed the acronym or initialism in brackets afterwards. For example:
According to a study performed by the National Aeronautics and Space Administration (NASA), dogs are actually aliens on a secret mission.
Any subsequent reference to it can use the acronym or initialism only, like this:
NASA continued to suggest that dogs merely lick us in order to mark us for death when they rise up. So contrary to what some people believe, licking is not a sign of affection.