The document discusses two types of relative clauses: defining and non-defining. Defining relative clauses provide essential information to the sentence and have no commas. Non-defining clauses provide non-essential information and are set off by commas. It provides examples of defining clauses using pronouns like who, which, that, where, when and whose. Non-defining clauses also use these same pronouns but are between commas. The document also briefly mentions other aspects like using an infinitive instead of a relative clause and emphasizing the subject of a relative clause.