Apostrophes can be used in a couple of ways. With contractions, use apostrophes to indicate that letters are missing.

When two words are written in shortened form, use an apostrophe to show that some letters are missing:

I've, it's, don't, rock 'n' roll, 'tis the season to be jolly. He is a ne'er-do-well.

Also use an apostrophe to mark missing numbers:

The class of '62. The Spirit of '76. The '20s.

Here are some tricky contractions.

It's / Its
Though it uses an apostrophe, it's is not a possessive. It's is the contraction for it is. Its is the possessive form of it.

Who's / Whose
Though it uses an apostrophe, who's is not a possessive. Who's is a contraction for who is. Whose is the possessive form of who.

Here are some examples:

I don't know whose coat it is. Find out who's there. It's time for dinner.

Taken from Cleaning Your Copy: Grammar, Style and More, a self-directed course at Poynter NewsU.

Take the full course

Have you missed a Coffee Break Course? Here's our complete lineup.