An apostrophe is used:

• to form contractions by showing the numbers or letters that have been left out.

EXAMPLE: '86 =1986
EXAMPLE: I am = I'm / we are = we're / he will = he'll / they would = they'd /do not = don't/I have = I've.

• to form the possessive of a noun.

Add 's to a single noun or name: uncle's pipe; George's girlfriend; dog's tail; Thomas's car.
Add 's to singular noun that end in –s: actress's role; princess's lover; rhinoceros's skin.
Add 's to plural nouns that end in –s: boys' bicycles; friends' houses; books' covers
Add 's to other plural nouns: children's toys; women's clothes; men's boots.
Add 's to a person's office or shop: I'll buy the pork at the butcher's. / I'll be visiting Tom's.
Add 's only after the second name: Jack and Jill's pail; Bonnie and Clyde's loot.

• to form the plural of abbreviations: many Dr.'s; many M.D.'s; many Ph.D.'s.

• for the plural of a number or letter: your p's; your c's; your 5's are too big.