Relative clauses provide extra information about nouns, functioning like adjectives. There are two types: defining clauses that identify the noun, and non-defining clauses that provide unnecessary but interesting information. Relative clauses can be introduced by relative pronouns like who, which, that, whose, or no pronoun, using where, why, or when instead. Defining clauses are not separated by commas while non-defining clauses use commas. Relative pronouns vary depending on if the clause refers to people or things.