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The apostrophe may be the simplest and yet most frequently misused mark of punctuation in English. Here we'll review six guidelines for using the mark correctly.

1. Use an Apostrophe to Show the Omission of Letters in a Contraction
Use the apostrophe to form contractions:

I'm (I am)
you're (you are)
he's (he is)
she's (she is)
it's* (it is)
we're (we are)
they're (they are)
isn't (is not)
aren't (are not)
can't (cannot)
don't (do not)
who's (who is)
won't (will not)
Be careful to place the apostrophe where the letter or letters have been omitted, which is not always the same place where the two words have been joined.

* Don't confuse the contraction it's (meaning, "it is") with the possessive pronoun its:

It's the first day of spring.
Our bird has escaped from its cage.

2. Use an Apostrophe with -s for Possessives of Singular Nouns
Use an apostrophe plus -s to show the possessive form of a singular noun, even if that singular noun already ends in -s:

Harold's crayon
my daughter's First Communion
Sylvia Plath's poetry
Dylan Thomas's poetry
today's weather report
the boss's problem
Star Jones's talk show
Victoria Beckham's husband

3. Use an Apostrophe Without -s for Possessives of Most Plural Nouns
To form the possessive of a plural noun that already ends in -s, add an apostrophe:

the girls' swing set (the swing set belonging to the girls)
the students' projects (the projects belonging to the students)
the Johnsons' house (the house belonging to the Johnsons)
If the plural noun does not end in -s, add an apostrophe plus -s:
the women's conference (the conference belonging to the women)
the children's toys (the toys belonging to the children)
the men's training camp (the training camp belonging to the men)

4. Use an Apostrophe with -s When Two or More Nouns Possess the Same Thing
When two or more nouns possess the same thing, add an apostrophe plus -s to the last noun listed:

Ben and Jerry's Cherry Garcia Ice Cream
Emma and Nicole's school project (Emma and Nicole worked together on the same project)
When two or more nouns separately possess something, add an apostrophe to each noun listed:
Tim's and Marty's ice cream (Each boy has his own ice cream.)
Emma's and Nicole's school projects (Each girl has her own project.)

5. Do Not Use an Apostrophe with Possessive Pronouns
Because possessive pronouns already show ownership, it's* not necessary to add an apostrophe:

yours
his
hers
its*
ours
theirs
However, we do add an apostrophe plus -s to form the possessive of some indefinite pronouns:
anybody's guess
one's personal responsibility
somebody's wallet
* Don't confuse the contraction it's (meaning, "it is") with the possessive pronoun its:
It's the first day of spring.
Our bird has escaped from its cage.

6. Generally, Do Not Use an Apostrophe to Form a Plural
As a general rule, use only an -s (or an -es) without an apostrophe to form the plurals of nouns--including dates, acronyms, and family names:

Markets were booming in the 1990s.
The tax advantages offered by IRAs make them attractive investments.
The Johnsons have sold all of their CDs.
To avoid confusion, we may occasionally need to use apostrophes to indicate the plural forms of certain letters and expressions that are not commonly found in the plural:
Mind your p's and q's.
Let's accept the proposal without any if's, and's, or but's.

Source: http://grammar.about.com/od/punctuationandmechanics/tp/GuideApostrophe.htm