Relative pronouns are used to either join two sentences together or to relate them to each other. For example: The famous singer who performs tonight is my sister.
The relative pronouns are:
Who - used for people (subject)
Whom - used for people (object)
Which - used for objects
Whose - used to show possession
Where - refers to places
When - refers to time
What - refers to things
That - refers to people, animals and things