When a noun shows possession, it is said to be in the possessive case.

Examples
1)    Ram’s book is on the table
2)    The dog bit Rita’s leg
3)    He lost his father’s purse
4)    Mrs. Sharma’s cat is called pussy
In these sentences, Ram’s, Rita’s, father’s and Mrs. Sharma’s are in the possessive case because each denotes possessor or owner of anything.

Remember
A noun or pronoun that denotes the possessor or owner of anything is said to be in the possessive.

Formation of possessive case
a)  The possessive case of a singular nouns formed adding an apostrophe (’) and S to it.
1)  The king’s palaces is situated on the river
2)  The boy’s father is an S.D.O.
3)  The horse’s leg was broken
4)  Mr. Das’s car was stolen
In these sentences the nouns in the possessive case are in the singular number. Their possessive cases has been formed by adding apostrophe (’) and S to it {king’s, boy’s, horse’s, Mr. Das’s}.
So for: the place of the king we write the king’s palace, the father of this boy we write the boy’s father, the leg of the horse we write the horse’s leg. The car of Mr.Das we write Mr. Das’s car. 

b) Possessive case of a Plural Noun that ends in S is formed by adding only an apostrophe (’) after it. No S is added to it.
1) The boys’ hostel in meant for boys
2) All these horses’ tails are long
3) The soldiers’ uniforms were beautiful
In these sentences, the nouns in the possessive case are in the Plural number and they end in S. their Possessive case has been formed by adding an apostrophe (’) to it.
So for
The hostel of the boys we write the boy’s hostel.
The tails of these horses we write these horses’ tails
The uniforms of the soldiers we write the soldiers’ uniforms

c) Possessive case of a noun in plural number that does end in S is formed by adding an apostrophe (’) and S after it.
1) We visited the men’s club
2) Children’s books are beautiful
3) The women’s collage is open today
In these sentences, the nouns in the possessive cases are in the plural number but they do not end in S. their possessive case has been formed by adding an apostrophe (’) and S after it.
So for
The club of men we write the men’s club.
The books of children we write children’s books.
The collage for women we write the women’s collage.