Generally, we don’t put full stops in acronyms or initialisms. Like FTSE, IRS and DNA.
If the short version’s more familiar than the full one, like BBC, FBI or FAQs, you don’t need to write it out.
If it’s not, or if you aren’t sure how well known it is, put it in full the first time you use it, then use just the short version from then on – International Atomic Energy Agency (IAEA). But avoid using it at all if you can – so you’d call the IAEA the Agency.
If you’re talking about people’s initials – like BB King or EM Forster – in London, we say no full stops, and no spaces.
But our New York office doesn’t mind the full stops, so there it’s B.B. King
People use the term ‘acronym’ to describe anything made up of initial letters, but most dictionaries define it as ‘a word’. So, NATO is an acronym (because it’s pronounced as a word) but DNA is an initialism. (Short for deoxyribonucleic acid.)
Some words we use normally are actually acronyms. Like scuba, which stands for ‘self-contained underwater breathing apparatus’.
An abbreviation (from the Latin brevis which means ‘short’) is a shortened form of a word or phrase, usually a letter or group of letters taken from that word or phrase. Like if you abbreviated ‘abbreviation’ to ‘abbrev’.
And no one knows what to call the ones whose pronunciation involves a combination of letter names and words, like JPEG or MS-DOS.