An abbreviation is a short form of a word or phrase that is used to represent the whole term. For example, etc. for etcetera, Sat for Saturday, Dec for December, Sonar for Sound Navigation and Ranging, UK for United Kingdom, etc. Abbreviations can be of many types; the most common ones are Acronyms and Initialisms.
An acronym is formed from beginning letters, syllables or parts of a word or phrase. It forms a new word and is usually, but not always, in all capital letters. An important point to remember is that acronyms are pronounced as words. It is a subset of abbreviation, i.e., all acronyms are abbreviations, but the reverse is not true.
NATO (North Atlantic Treaty Organization)
Scuba (Self-contained underwater breathing apparatus)
Radar (Radio Detection and Ranging)
OPEC (Organization of the Petroleum Exporting Countries)
AIDS (Acquired Immune Deficiency Syndrome)
ELISA (Enzyme Linked Immuno-Sorbent Assay)
RAM (Random Access Memory)
LASER (Light Amplification by Stimulated Emission of Radiation)
NASA (National Aeronautics and Space Administration)
Interpol (International Criminal Police Organization)
An initialism is another type of abbreviation that is made up of the initial letters of the name or phrase. It is different from an acronym as the former is pronounced one letter at a time, i.e., each letter is read separately, and not as a word.
BBC (British Broadcasting Corporation)
FBI (Federal Bureau of Investigation),
CIA (Central Intelligence Agency)
HTML (Hyper-Text Markup Language)
IBM (International Business Machines)
DVD (Digital video disc)
BTW (By the way)
UN (United Nations)
USA (United States of America)
CD (Compact Disc)