In fact shortening is a kind of abbreviation.
According to MSN Dictionary 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 to read more about abbreviation.