The apostrophe is a punctuation mark, and sometimes a diacritical mark, in languages that use the Latin alphabet and some other alphabets. In English, the apostrophe is used for three basic purposes:
The marking of the omission of one or more letters, e.g. the contraction of "do not" to "don't"
The marking of possessive case of nouns
Use as a single quotation mark
It is also used in a few distinctive cases for the marking of plurals, e.g. "p's and q's" or Oakland A's. From Wikipedia
