A defining relative clause (an information essential to the meaning of the main sentence. It is not put in commas and is introduced with who, which, that, whose, where, when or the season (why).
The man who bought our car lives next door.
A non-defining relative clause (a non-identifying relative clause) gives extra information and is not essential to the meaning of the main sentence. It is put in commas and is introduced with who, whom, which, whose, where or when. The woman, who was ninety year old, walked very slowly. That cannot be used to introduce a non-defining relative clause