Apostrophe

The apostrophe (Ↄ, ↄ, ’), also known as the apostrophus, is a punctuation mark used in writing. It is also a diacritic.

In English, it has two jobs:Quirk, Geenbaum, Leech & Svartvik, A comprehensive grammar of the English language. Longman, London & New York. p985 
 To show where one or more letters have been left out, as in the abbreviation (contraction) of do not to don't.
 To show the possessive case, as in the cat’s whiskers.

 Examples 
 Its versus it's 
The most common grammatical mistake in written English is to put it's where its is correct. 

Its: a possessive adjective and pronoun of the personal pronoun it. 
The cat chased its tail. (correct)
The tyre lost it's grip. (wrong)

It's: a contraction of the verbal phrase it is or it has.
It's mine. (correct; check: It is mine)
It's been here. (correct; check: It has been here)
The cat chased it's tail. (wrong; cannot be expanded to it is)

The same applies to yours, theirs and ours because these are also possessive adjectives of personal pronouns.
The colour is ours.
That book is hers (or his).
Theirs was the responsibility.

 Possession 
Apostrophes are also used to show something belongs to someone (or something). Again, correct uses can be expanded:
 Mike's car. (correct; the car that belongs to Mike)
 The dog's ball. (correct; the ball that belongs to the dog)
Those dog's are large. (wrong; here "dogs" is a plural word)

 The intrusive apostrophe 
Comes in plurals which don't (do not) need it. Do not put an apostrophe in word ending in s, such as a plural. Put an apostrophe, or 's, at the end of the word instead.
Mr. Jones' hat or Mr. Jones's hat. (both correct)
Both of my parents' birthdays. (correct)
CD's and DVD's (wrong; not possessive. See "Plural" section below)
Apple's and pear's (wrong; not possessive)

 Writing dialogue or titles 
Apostrophes are also used when other words are shortened, as in slang:
Go get 'em tiger! or Li'l Bow Bow.
This is just a version of the abbreviation function.

 Plural 
To make a word that doesn't (does not) usually exist as a plural into a plural, an apostrophe is occasionally used. See these examples:
 How many A's did you get this year? Here it is wrong because it is not needed.
 The poll received many yes's and very few no's. Here it is sensible because without it the words 'yess' and 'nos' look quite peculiar. However, you may find 'yeses' and 'noes' are appropriate as plurals. General rule: if an apostrophe is not needed, do not use it.

 References 

Category:Punctuation
Category:Grammar