Initialisms are very similar to acronyms in that they are made up of letters of some name or phrase, usually the first letter of each word as is common with acronyms. The difference between an acronym and initialism is that the abbreviation formed with initialisms is not pronounced as a word, rather you say the individual letters, such as FBI (Federal Bureau of Investigation), CIA (Central Intelligence Agency), and DVD (Digital Video Disk*).
And, of course, if it’s just a shortened form of a word, like “ex.” for “example”, then it’s neither an acronym nor an initialism, rather just an abbreviation.