What is the difference between an acronym and an initialism?

Both acronyms and initialisms are made up of the first letter or letters of the words in a phrase. The word acronym typically applies when the resulting thing can be read as a word; for example, radar comes from "radio detection and ranging" and scuba comes from "self-contained underwater breathing apparatus." The word initialism only applies when the resulting thing is read as an abbreviation; for example DIY, which comes from "do it yourself," is pronounced by saying the names of the letters. Note that the word acronym is also sometimes used to mean "initialism."

What is the difference between an acronym and an abbreviation?

An acronym is a kind of abbreviation. Abbreviations can be shortened forms of any kind. For example, appt is an abbreviation of appointment, and ASAP is an abbreviation of as soon as possible. ASAP, however, also qualifies as an acronym because it is made up of the initial letters of the phrase it comes from: as soon as possible.

Is OK an acronym?

OK is technically an acronym. It comes from the phrase "oll korrect," a humorous alteration of "all correct."