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that, which, who

As a general rule the relative conjunction which introduces nonrestrictive clauses; that introduces restrictive clauses.

If dropping the clause would change the meaning, use that without commas.
            Jokes that are off-color do not belong in the magazine.
             This joke, which is off-color, does not belong in the magazine.

If the clause can be dropped and all one loses is additional information, use which and set off the clause with commas.

Use who—not that or which—when referring to people.