We use:

• who and whom for people
• which for things
• that for people or things.

The relative pronoun is the subject/object of the relative clause, so we do not repeat the subject/object:

Marie Curie is the woman who she discovered radium.
(who is the subject of discovered, so we don't need she)

This is the house that Jack built it.
(that is the object of built, so we don't need it)

2.  We also use relative clauses to give more information about a person, thing or situation:

Lord Thompson, who is 76, has just retired.
We had fish and chips, which I always enjoy.

I met Rebecca in town yesterday, which was a nice surprise.

whose and whom

We use whose as the possessive form of who:

This is George, whose brother went to school with me.