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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

Whose is used to show possession. It has the same meaning as other possessive adjectives such as his, hers, its, their, etc.

–There's the man whose house we bought.

–I have a book whose story is fascinating.

Whose modifies people but can also be used with things.

You should learn how to combine short sentences using whose:

–The woman is a talented artist. I saw her paintings.

–The woman whose paintings I saw is a talented artist.

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For people, you will use either who or whom. Who is usually used instead of whom in colloquial speech, even though it is technically incorrect:

–The person who they saw was sick. (informal)

–The person whom they saw was sick. (formal)

–There's the driver who the police arrested. (informal)

–There's the driver whom the police arrested. (formal)