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, like above. (This is the easy part.)

Chapter 6: Answers to the Real Essay Questions