The most common relative pronouns are :
who- which- that- whom- whose
We use who to represent defining and non-defining relative clauses
A doctor is a person who works in hospital.
Whom is used to refer to people when the person is the object of the verb
He is the person whom I admired most.
Whose is used to express possession by people and animals.
The man whose car is red, is my teacher.
Which refers to objects and animals. It's used to express defining and non-defining relative clauses
This is the house which I bought yesterday.
That is used to refer to animals or things and persons. We can use it to replace who- which - whom
The 7.15 is the flight that you need to get