An abbreviation is a truncated word; an acronym is made up of parts of the phrase it stands for and is pronounced as a word (ELISA, AIDS, GABA); an initialism is an acronym that is pronounced as individual letters (DNA, RT-PCR).

What are examples of initialism?

Initialisms are made from the first letter (or letters) of a string of words, but can't be pronounced as words themselves. Examples include FBI, CIA, FYI (for your information), and PR (public relations).

Why are initialism used?

Essentially, initialisms are shorter forms of words or phrases that can come in handy when you need to repeat the same word or phrase a number of times throughout the same piece of writing. They are pronounced as a series of letters.