Any shortened form a word is an abbreviation. Acronyms and initialisms are abbreviations, but not all abbreviations are acronyms or initialisms.
An acronym is a special kind of abbreviation that uses (usually) the first letter of each word that it represents to form an all-letters-capitalized abbreviation (e.g., North Atlantic Treaty Organization is NATO). A true acronym can be pronounced as a word, such as OPEC, NATO, and HAZOP.
An initialism is just like an acronym, BUT it CANNOT be pronounced as a word. Instead, you have to say the individual letters (e.g., FBI, CIA, PTA) when you pronounce it.
Still, even though technically there is a difference between acronyms and initialisms, we treat them the same and we normally just call them all acronyms.