Hello, There! Well, I found this question at the Wikianswers site.
There you can ask a question or find interesting questions like this one. It is indee a very interesting question...... I found out that apostrophes have three uses:
1) to form possessives of nouns
a)add 's to the singular form of the word (even if it ends in -s): For example: Doris's class, the girl's book.
b) add 's to the plural forms that do not end in -s:
the children's game, the mice's tails
c) add ' to the end of plural nouns that end in -s:
houses' roofs , three friends' letters
d) add 's to the end of compound words:
my brother-in-law's money ?
e) add 's to the last noun to show joint possession of an object:
Todd and Anne's apartment
2) to show the omission of letters
Apostrophes are used in contractions. A contraction is a word (or set of numbers) in which one or more letters (or numbers) have been omitted. The apostrophe shows this omission. Contractions are common in speaking and in informal writing. To use an apostrophe to create a contraction, place an apostrophe where the omitted letter(s) would go. Here are some examples:
don't = do not
I'm = I am
he'll = he will
who's = who is
shouldn't = should not
didn't = did not
could've= could have
'60 = 1960
3) to indicate certain plurals of lowercase letters.
Forming plurals of lowercase letters Apostrophes are used to form plurals of letters that appear in lowercase; , e.g. "three ps" versus "three p's." To form the plural of a lowercase letter, place 's after the letter. Here are some examples:
p's and q's = a phrase indicating politeness, possibly from "mind your pleases and thankyous"?
Nita's mother constantly stressed minding one's p's and q's.
three Macintosh G4s = three of the Macintosh model G4
There are two G4s currently used in the writing classrom.
many &s = many ampersands
That printed page has too many &s on it.
the 1960s = the years in decade from 1960 to 1969
The 1960s were a time of great social unrest.
Don't use apostrophes for possessive pronouns or for noun plurals.
* taken from http://wiki.answers.com/Q/When_do_you_use_an_apostrophe
* Image taken from: http://onthemarkwriting.files.wordpress.com/2007/08/apostrophe.png