Who refers to people: "The boy who wrote the book stands over there."
That refers to things or groups of people (not a specific person):" Mary is on the team that won the competition. " "Chairs that have four legs are more stable."
That can be used for people as well, but this is considered informal and is not fashionable. In spoken English you will however hear this.
Which refers to groups or things as well, but in a slightly different way: "The chair over their, which has three legs, was painted red by my uncle."
In this sentence, the part "which has three legs" is additional information that can be left out and you would still understand the sentence. In these cases, American English says that you shall use "which". In British English the use of "which" or "that" is less strict, and both words can be used.
Whom is used in the following way: "My uncle whom I love very much paints many chairs." It is "I" who does the loving, "I" is the subject and "my uncle" is the loved party- the object. If you make the sentence into a question and the answer would be "him", then use whom. If the answer is "he", then use who. E.g.: Who do you love very much? Him -> Therefore: The uncle whom I love very much.
Compare this with the following sentence:
"It is my uncle who paints the chair." Who is the person, the subject, that paints the chairs. Question: Who paints the chairs? It is he...
Whose is used to show possession (ägande/genitive): "Over there is the artist whose favourite painting was exhibited at the museum."
EXERCISES
Exercise 1 - who/which/that
Exercise 2 - who/which/that/whose
Exercise 3 - who/which/whose
Exercise 4 - who/which/that/where
Exercise 5 - who/which
Exercise 6 - who/which/whose
Exercise 7 - who/whose