We use who when referring to people. We use which or that when referring to things or to animals.

• Alice sings well. She has won a prize.
• Alice, who sings well, has won a prize.
• The airplane was grounded. It had engine trouble.
• The airplane, which had engine trouble, was grounded.
• The dress was bought yesterday. It is made of expensive cloth.
• The dress, which was bought yesterday, is made of expensive cloth.

Notes:

We can use that in place of whom when referring to people.

• The man that the police is looking for is dead. (= The man, whom the police is looking for, is dead.)

Notice that no commas are necessary, when we use that.

Related posts:

1. Joining two sentences with a preposition and which
2. Forms of the Relative Pronouns
3. Relative Pronouns
4. Use of the Relative Pronouns Who and Which