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.
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.
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:
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.