* who (refers to people)
* whom (object form of who)
* whose (shows possession)
* which (refers to things)
Example sentences:
* Who is the person sitting next to you?
* The woman whom I saw at the party was my aunt.
* The book whose cover is blue is mine.
* The car which is parked in the driveway is my dad's.
Note: "That" can also function as a relative pronoun, but it's often considered more informal.