In fact shortening is a kind of abbreviation.

According to [MSN Dictionary][1] There are **four main kinds of abbreviations**:

 - **Shortenings** of words usually consist of the first few letters of the full form and are usually spelled with a final period when they are still regarded as abbreviations

> *cont. = continued*

> *bus = omnibus*

> *taxi = taxicab*

> *zoo = zoological garden*

> *bike = bicycle* 

 - **Contractions** are abbreviated forms in which letters from the middle of the full form have been omitted.

> *Dr. = doctor*

> *St. = saint or street*

> *can't = cannot*

> *didn't = did not*

 - **Initialisms** are made up of the initial letters of words and are pronounced as separate letters.

> *CIA (or C.I.A.)*

> *NYC*

> *pm (or p.m.)*

> *U.S. (or US)*

 - **Acronyms** are initialisms that have become words in their own right, or similar words formed from parts of several words. **They are pronounced as words rather than as a series of letters.**

> *AIDS*

> *laser*

> *scuba*

> *UNESCO*

Please Visit this [link][2] to read more about abbreviation.


  [1]: http://dicary.msn.com/
  [2]: http://encarta.msn.com/encnet/features/dictionary/DictionaryResults.aspx?lextype=3&search=abbreviation