1 Answer

The apostrophe ( ’ although often rendered as ' ) is a punctuation mark, and sometimes a diacritic mark, in languages that use the Latin alphabet or certain other alphabets. In English, it serves three purposes:
The marking of the omission of one or more letters (as in the contraction of do not to don’t).
The marking of possessive case (as in the cat’s whiskers).
The marking as plural of written items that are not words established in English orthography (as in P’s and Q’s, the late 1950’s). (This is considered incorrect by some; see Use in forming certain plurals. The use of the apostrophe to form plurals of proper words, as in apple’s, banana’s, etc., is universally considered incorrect.)
According to the Oxford English Dictionary (OED), ‘apostrophe’ comes ultimately from Greek ? ?π?στροφος [προσ?δ?α] (h? apóstrophos [pros?idía], “[the accent of] ‘turning away’, or elision”), through Latin and French.
The apostrophe looks the same as a closing single quotation mark, although they have different meanings. The apostrophe also looks similar to, but is not the same as, the prime symbol ( ′ ), which is used to indicate measurement in feet or arcminutes, as well as for various mathematical purposes, and the ?okina ( ? ), which represents a glottal stop in Polynesian languages.

Read more here>>>http://en.wikipedia.org/wiki/Apostrophe 

Rules>>>http://www.grammarbook.com/punctuation/apostro.asp