The document discusses relative clauses, which are formed by joining two sentences using a relative pronoun like who, which, that, when, or whose. There are two types of relative clauses: defining relative clauses, which provide essential information about a noun and are not set off by commas, and non-defining relative clauses, which provide extra information and are set off by commas. The document provides examples and guidelines for using relative pronouns in defining and non-defining relative clauses.