There are five relative pronouns that are used to begin a relative clause: who, whom, whose, which and that.
Who – used to introduce a relative clause that relates to a person as the subject.
The person who called me was my mother.
My mother,who is very nice, calls me all the time.
Whom – used to introduce a relative clause that relates to a person as the object.
That person, whom I did not like, smelled like cheese.
Whose – used to introduce a relative clause that relates to possession.
The woman is looking for a new boyfriend whose personality is kind and generous.
The woman, whose current boyfriend is quite mean, is planning to get a new boyfriend.
That – used to introduce a defining relative clause that refers to a thing or a person.
The person that is in front of you is very tall.
The iPad that is on the desk is broken.
Which – used to introduce a relative clause that refers to a thing.
The computer which I bought you is broken.
The computer, which is sitting on the table over there, is broken.
Read more about relative clauses here to learn how and when to use these pronouns.