Term: Non-defining Relative Clauses
My sister, who lives in France, is coming to stay with me next week. ('who lives in France' is not essential, which means that I only have one sister and she does not need to be defined by the relative clause)
'Who' and 'whose' are used for people. 'Which' and 'whose' are used for things. 'That' cannot be used in a non-defining relative clause.
See Also: Defining Relative Clause; Relative Pronoun Related Article: Relative Clauses - Learn about Relative Pronouns in Non-Restrictive Clauses (Non-Defining clauses) and Restrictive Clauses (Defining clauses).
Category: Relative Pronouns