• Who and whom are used instead of people.
• That and which are used instead of things, places, and ideas.
• That is sometimes also used instead of people.

We also have a special possessive for relative clauses: whose

I don't know whose book that is.

• The man who is at the table is waiting to order.
• I don't know that woman to whom you are waving.
• The dog is eating that pizza which was left on the counter.

Who vs. Whom

Whom is more formal and is used after a preposition. "to whom it may concern"

If you are not sure if you should use who or whom check to see if it is following a preposition. If it is not then go ahead and use who.