This That and Other Things

An abbreviation is a shortened form of a word or phrase.

1. An abbreviation is not pronounced as it is written. Examples: We pronounce Dr as Doctor, Jan. as January, abbr. as abbreviation.
2. An abbreviation usually ends in a full stop. Examples: subj., etc., Pres., adj., Dec., Fri., Univ.
3. If the abbreviation ends with the last letter of the word, you do not use a full stop. Examples: Dr, Mr, Mrs, St, Ave, Sgt
4. If the initial letters of words make up an abbreviation, you do not use a full stop. This is called an initialism and the letters are pronounced separately. Examples: SPCA, UK, HIV, BBC, CIA, UN, CD
5. If the initials of a group of words form a new word, it is called an acronym. The word is pronounced as spelt and no full stops are used. Examples: NATO, AWOL, Aids, Scuba, Laser, Asap (See Seven Rules for Acronyms)

http://writerswrite.co.za/abbreviations-five-tips-for-writers