There are two types of relative clauses: defining and non-defining. Defining clauses are essential to the meaning of the sentence, while non-defining clauses provide extra information that is not essential. Defining clauses do not have commas, while non-defining clauses always appear between commas. Relative pronouns like who, that, and which introduce both defining and non-defining clauses, while adverbial pronouns like when, where, why, and whose only introduce non-defining clauses.