Possessive Apostrophes are added to the end of a noun to show possession‏‎.

This car belongs to John. It is John’s car.

That dog belongs to that girl. It is that girl’s dog.

Note, a Possessive Apostrophe is sometimes known as a Saxon Genitive.

Plural Nouns

If there is a plural noun‏‎ which ends in an -s, then we add just an apostrophe after it (no extra -s). In this case there are many girls and boys who each have bikes:

the girls’ bikes

the boys’ bikes

In this next example we are referring to 1 lady and her bag:

the lady’s bag

However, in this example we are referring to many ladies and all their bags:

the ladies’ bags

Note that for plural nouns which do not end in -s we use the apostrophe along with an -s:

The men’s room.

The women’s committee.

Proper Nouns

Likewise, proper nouns‏‎ take an apostrophe -s to show possession:

That is Shakespeare’s play.

This is Napoleon’s hat.

There is some confusion when it comes to proper nouns which end in -s. In most cases we can add an apostrophe -s.

This is Ross’s book.

That’s Francis’s home.

However, if the proper noun ends in -es then we normally use an apostrophe without the -s:

Socrates’ philosophy

Euripedes’ works

Having said this, there are different opinions about using an -s with any proper noun which already ends in -s or an -s type sound. These are all acceptable:

Bridget Jones’ diary.

Bridget Jones’s diary.

Jesus’ sermons.

Jesus’s sermons.