Hold off before calling the priest and bringing in the Holy Water! I am not talking about that kind of possession with ghosts and demons. I am talking about showing possession when you are writing and using apostrophes.

Apostrophes are mighty little pieces of punctuation that are mostly unique to the English language. The English language uses apostrophes to show possession, unlike most other languages. Other languages omit apostrophes and show possession in another way.

For example, in the Spanish language, apostrophes tend to be omitted.  Let’s take Sam and his Dog:

When showing possession, the English language would tell us to say, “Sam’s dog”.   However, the Spanish language tells us to say “The dog of Sam” or “El Perro de Sam”.

So the question becomes: how do you use apostrophes? Well, there are some basic rules.

Rule #1: Possession for a singular person.

To show possession for a single person, object, etc. you would add an apostrophe and an “s” to the end.  For example:

“The toy belonging to the cat …”, we can say “The cat’s toy …”. We can shorten the fragment down to three words using the  apostrophe with an “s”

Rule #2: Plural Possessives.

Okay, so we need to show possession for multiple things. We need to pay particular attention to where we put the apostrophe and where we put the “s”. In plural possessives the apostrophe comes AFTER the “s”. For example:
“The food belonging to the group of babies” would be turn into “The babies’ food”.

Note: If the food belongs to one baby, the phrase turns to “The baby’s food”.

A plural possessive may seem easy enough, but it gets much more difficult when you’re dealing with compound plural possessions, or when two people possess the same item.

Now this gets tricky, so pay attention:

When two people own a thing together, you would use only one apostrophe. For example: “John and Jane’s wedding was so elegant”, or “Sarah and Doyle’s cat was in the tree”.

But if two people own items separately, then you need to use two apostrophes. For example: “Nate’s and Amy’s boats were stuck in the mud” or “Judy’s and Sami’s books got wet in the rain”.

Rule #3:  Apostrophes are used only for contractions or to show possession.

It is important that you only use apostrophes on nouns to show possession. Plural nouns do not get apostrophes, although some people use them that way.

For example:

Correct: The cats are sleeping.
Incorrect: The cat’s are sleeping.

Note: The cat’s do not own the “are”.

For further reference: http://www.dummies.com/how-to/content/using-apostrophes-to-show-possession.html