Use “who” only to refer to people.
Maria and Julie are two sisters that equally share household responsibilities.
Maria and Julie are two sisters who equally share household responsibilities.
Use “which” and “that” to refer to things.
Use “that” in restrictive clauses–clauses that restrict the meaning of the noun in some way.
The house that caught fire is down the block from mine.
Here, we need to know that the house caught fire in order to know which house we are talking about; in other words, the identity of the house is restricted by the fact that it is the one that caught fire, rather than any of the other houses that are also down the block from mine.
Use “which,” on the other hand, to refer to non-restrictive clauses–clauses that tell us something incidental about a subject, but don’t restrict the meaning of that subject. In other words, use “which” for non-essential information.
The house, which caught fire last year, is painted blue and yellow.
Here, the information that the house caught fire is merely an aside; we don’t need to know it in order to know that we are talking about the blue and yellow house.
Notice that the clause containing “that” is not set off by commas, whereas the clause containing “which” is set off by commas. Commas around a clause are another clue that the clause is non-restrictive, an indication to use “which.”
Here are a few more examples.
The candidate that who receives more votes will win the election.
The rock that she threw at the window is still on the front lawn.
The rock, which she threw at the window, was so heavy she needed two hands to hold it.