The Possessive case is used to express that something belongs to someone or to show a relationship between two or more people. We show this ownership by adding an apostrophe and an s to the noun if the noun is a person or an animal. We use the word "of" if there is a relation between two or more things.
With persons and animals:
This is Tom's bike.
This is Kathy's mother.
These are the boys' bikes.
This is the girls' mum.
's with irregular plural forms: These are our children's cell phones.
's or s' with people's names ending in "s": This is Charles' / Charles's car.
To express an ownership for two or more people put the 's to the final name: This girl is Ken and Bob's sister.
With things: of
The colour of the table is black.
The second chapter of this book.