Taken from the Apostrophe Protection Society:
The rules concerning the use of Apostrophes in written English are very simple:
1. They are used to denote a missing letter or letters, for example:
I can't instead of I cannot
I don't instead of I do not
it's instead of it is
2. They are used to denote possession, for example:
the dog's bone
the company's logo
Jones's bakery (but Joneses' bakery if owned by more than one Jones)
... but please note that the possessive form of it does not take an apostrophe any more than ours, yours or hers do
the bone is in its mouth
... however, if there are two or more dogs, companies or Joneses in our example, the apostrophe comes after the 's':
the dogs' bones
the companies' logos
3. Apostrophes are NEVER ever used to denote plurals! Common examples of such abuse (all seen in real life!) are:
Banana's for sale which of course should read Bananas for sale
Menu's printed to order which should read Menus printed to order
MOT's at this garage which should read MOTs at this garage
1000's of bargains here! which should read 1000s of bargains here!
New CD's just in! which should read New CDs just in!
Buy your Xmas tree's here! which should read Buy your Xmas trees here!
Note: Special care must be taken over the use of your and you're as they sound the same but are used quite differently:
your is possessive as in this is your pen
you're is short for you are as in you're coming over to my house
In short, an apostrophe is something which many people don't know how to use correctly.
Jesus never had 12 apostrophes nor a last KFC supper.