A relative clause is a subordinate clause that provides information about the antecedent, or the word it refers to. There are two types of relative clauses: defining relative clauses, which specify the antecedent and are not separated by commas, and non-defining relative clauses, which add extra information and are separated by commas. Relative pronouns like who, which, that, when, and where introduce the relative clauses and join them to the antecedents.