The apostrophe is used to show ownership. Most of the time, it presents no confusion:

Bob’s bassoon
The woman’s finger
My son’s toys

The tricky part is using an apostrophe when the owner is plural.

RULES FOR APOSTROPHES

1. If the plural noun doesn’t end in -s, add an apostrophe and -s, as shown above. (This is the easy part.)

the car’s axles
the bacteria’s growth
the mice’s hairballs

2. If the plural noun ends in -s, just add an apostrophe.

the babies’ bottoms
the horses’ hooves
the politicians’ promises

3. If the word is a proper noun that ends in -s, add an apostrophe and an -s. (This is the part people get wrong). Use ONLY with proper nouns. All other plurals should follow the rule above.

Yeats’s poem
Ross’s riddle
Chris’s crisis

An alternate way to express the possessive for proper nouns already ending in -2 is to simply put the apostrophe at the end of the -s.

Yeats’ poem
Ross’ riddle
Chris’ crisis