The Attributive Clause
who, whom, whose, that, which, as  
  when, where, why
Looking at examples:
The number of people who / that lost homes reached as many as 250,000.
The boy ( who / whom / that ) we saw yesterday  was John’s brother.
It sounded like a train which / that  was
going under my house.
The car ( which / that ) my uncle had just bought was destroyed in the earthquake.