Relative clauses provide additional information about a person or thing mentioned in the main clause of a sentence. Defining relative clauses are essential for identifying the person or thing, while non-defining relative clauses provide extra context using commas. Relative pronouns like who, which, that, and whose are used to join the two clauses, and the pronoun can sometimes be omitted for objects. The type of relative clause impacts whether it refers to a specific or non-specific person or thing.