This document discusses defining and non-defining relative clauses. Defining relative clauses provide essential information to identify the person or thing being described, and do not use commas. Non-defining relative clauses provide extra, non-essential information and are set off by commas. Relative pronouns like who, that, which, and whose are used differently in defining and non-defining clauses. The document provides examples of sentences using defining and non-defining relative clauses with people, things, animals, time, and place.