Who refers to people. Which refers to specific animals or things. That refers to animals, things, or people as a class. the object (direct or indirect) form of who is whom. Which and that both have the same form whether subject or object.
- The man who called you is here.
- The girl whom you saw is my sister.
- To whom did you give the key?
- Is this the book which you ordered?
- The magazine which is on the table is old.
- The native Americans that lived here were called Sloux.
- These are the colors that we like.