There you are, faced again with whether to use the apostrophe or not. It is a tricky thing - sometimes used for plural forms of words, but mostly not. Certainly used to denote possession . . . except with those pesky possessive pronouns. What ARE the rules? you ask. 

Well, the teacher in me has to answer. Let's see how we can break it down:

1. Use an apostrophe to show the omission of letters or numbers, a contraction.

Examples: 
It's a shame. (Stands for: It is a shame.)
That happened in '80. (Stands for: That happened in 1980.)

2. Use an apostrophe to show the plural form of letters and numbers.

Examples:
There are two s's in that word.
She dots all her i's and crosses all her t's.
My son can say his 1's, 2's, and 3's in five different languages.

3. Use an apostrophe to show the plural form of a few unusual words.

Examples:
She mailed all the thank-you's so late that we were embarrassed.
The contract had so many "that's" in it, that we felt uncomfortable about signing it.

4. For the most part, you do NOT use the apostrophe to show plural form. 

Examples:
There were many heros (or heroes) in my hometown.
Victories for the enemy numbered in the hundreds.

5. Use an apostrophe to show possession.

Example:
That is Sandra's book.
NOTE: You usually have a noun after the word that shows possession. Before you use the apostrophe, stop and ask yourself if there is a word in that sentence that denotes something belonging to someone. If not, then rethink using an apostrophe.

6. Do NOT use an apostrophe to show possession with pronouns. "Its" is often confusing to writers. Its is possessive, just like his, hers, and ours. None of the other possessive pronouns have apostrophes, so that is a good way to remember that its doesn't have one either. ("It's" can only mean one thing: it is.)

Examples:
The bridge was elegant - a symbol of its own strength and beauty.
The victory was ours.

7. Use an apostrophe before the s when showing the possessive form of singular nouns.

Examples:
The child's mother panicked when she could not find her son.
The witness's memory was vague on that point.

8. Use an apostrophe after the s when the word is plural and possessive, if its plural form is regular.

Examples:
Our creditors gave us two weeks' notice.
The Joneses' cars were all stolen.
You have heard both the girls' stories.
NOTE: The noun following a plural possessive is usually - although not always, as evidenced in the first example - plural also, i.e. two girls, two stories and two families of Joneses and two cars.

9. Use an apostrophe before the s when the word is plural and possessive but its plural form is irregular.

Examples:
The children's recess is about to start.
The women's cries were heard.

10. Use an apostrophe only after the second name when the possession belongs to two people and they are mentioned in the sentence at the same time.

Example:
That computer program is Jane and Sam's.
NOTE: One computer program belonging to two people, i.e. joint ownership.

It's a crazy thing, this apostrophe, how to know when it's its or it's, and keeping up with the Jones' or the Joneses' way of life is getting more and more complicated all the time!