Who, which and that are relative pronouns. They are usually used in dependent clauses introduced by a main clause:

--I tipped the waiter who (that) served us.
--We thanked the people who (that) helped us.

Note that in the above two examples who and that are substituted for one another. Even though who is more grammatically correct than that in reference to a person or persons, that is heard often in speech.

Who is used only for people. That can be used to refer to a person or thing. Which is used only for things:

--The car, which we rented, doesn"t work