The apostrophe has three uses:
•  1. to form possessives of nouns.
•  2. to show the omission of letters
•  3. to indicate certain plurals of lowercase letters

•  Add 's to the singular form of the word, even if it ends in "s"
  The Owen's Beach House
  Chris's Hot Dish

•   Add 's to the plural forms that do not end in "s"
  The Grandchildren's Birthday Celebration
  The Howard's Kickasserole

•   Add ' to the end of plural nouns that end in "s"
  The Smith Kids' Party
  Our Friends' Party

•   Add 's to the end of compound words:
  Step-Mom's Specialty
  Great-Granddad's Pork Butts

•   Add 's to the last noun to show joint possession of an object
  Todd & Anne's Barbecue
  Bobo & Pooke's Chow

•   Apostrophes are NOT used for possessive pronouns or for noun plurals, including acronyms.
  UCLA Student Body Lunch
  USC Faculty Celebration

•  Showing Omission of Letters and Numbers
  BBQ'n = barbecuing
  doin' = doing
  don't = do not
  I'm = I am
  cookin' = cooking
  '05 = 2005 or 1905