It's the turn of the apostrophe.

Contraction, possessive, omission, non-standard English, all use the apostrophe. Lynne Truss in 'Eats, Shoots & Leaves' claims that the apostrophe is the frantically multi-tasking female of the punctuation world, while the full stop is the lumpen male.
The placement of this little lick has caused much debate, and humor, over the centuries.
Let's look at examples:
Contractions
In speaking we run words together and these are known as contractions e.g.
aren't - are not
can't - can not
won't - will not
let's - let us
I'd - I would
don't - do not
Then there's the trickier and often abused it's, they're, and you're.
It's - is a contraction to replace the 'i' of is. It's only used for this no other. If you're unsure put the 'is' after the 'it' and you'll soon find out if it makes sense or not .e.g. The dog bit it's tail. should read - The dog bit its tail.
The same goes for they're, which only stands for they are. Not to be confused with their(possession), and there (place).
And you're, which stands only for you are. Not to be confused with your (possessive).

Possessive Apostrophes
These fall into two categories. - Singular possessive and plural possessive.
Singular possessive
You simply add- 's to the singular noun e.g. boy's hat = a hat belonging to one boy; the baby's bib
If the singular noun ends in an 's' - modern usage add's the apostrophe alone but no - s's - e.g.
H.G. Wells = H.G.Wells' books. ( older usage is H.G.Wells's books)
Plural Possessive
You add - 's - to the plural noun e.g. The children's coats. The men's ideas made us laugh.
If the plural noun ends with an 's' then just add the apostrophe e.g.The girls' hats.
In general in writing it is acceptable to show the possessive case of inanimate objects e.g. the cuff of my pants ( not my pant's cuff); the seat of my pants (not my pant's seat)
But there are exceptions that indicate time and quantity:
thirty days' notice
a week's pay
an hour's walk
for pity's sake
a day's travel
a month's travel
three pounds' worth

And the less common- 's - after the silent 's' at the end of a word e.g. Arkansas's capital city is Little Rock.
Nowadays the - ' - is usually omitted in corporate names e.g. Barclays Bank, or Chambers English Dictionary
In abbreviations it is becoming more common to omit the possessive apostrophe (and points) e.g.V.I.P.'s vs VIPs; M.P.'s vs MPs
An apostrophe is used to indicate the plural of single letters e.g. Mind your p's and q's. He was one to dot the i's and cross the t's.
They are also used to indicate the plural of a word when they are not used to signify their normal meaning e.g. His speech was full of if's and but's. The manuscript was edited to take out all the got's.
Possessive determiners and pronouns DO NOT HAVE APOSTROPHES
determiners are: my/our, your/your, his/their, her/their, its/their
pronouns are: mine/ours,yours/yours, his/theirs, hers/theirs,its/theirs

Apostrophes of omission
These occur when significant letters of a word are left out and is often used in idiom e.g.
We can't go to Jo'burg today. "It was 'im that got the cat-o'-nine-tails, and he'd be dead if we 'adn't stopped 'im."
Words like fridge, phone, telly,pram, cello, nuke, photo, etc. no longer need the apostrophe to signify the letters absent.

Non-standard English apostrophe.
When writing old English and classical names the apostrophe is used with out a following s.
e.g. Xerxes' battles, Socrates' ideas, Achilles' heel, Archimedes' bath
The same goes for place names in modern usage e.g.Barbados' capital city is Bridgetown.
Names ending in an 'iz' sound e.g. Bridges' score, Moses' tablets and lastly Jesus' disciples.
Still, there would be little complaint if you added the 's' after the apostrophe for any of these, with the exception of Jesus.

Hope these help your apostrophe dilemma... my references were Rudolf Flesch and A.H. Lass - The Classic Guide to Better Writing; Write On by Richard Bell and Pauline Bently; Eats, Shoots & Leaves by Lynne Truss.