Relative clauses are non-essential parts of a sentence. They may add meaning, but if they are removed, the sentence will still function grammatically. There are two broad types of relative clauses in English. It is important to distinguish between them because it affects the choice of pronoun used to introduce the clause. There is a more detailed page about preposition placement in relative clauses.

A defining or identifying clause tells us which specific person or thing we are talking about in a larger group of people or things. If a defining relative clause is removed, the meaning of the sentence changes significantly. A defining relative clause is not separated from the rest of the sentence by commas or parentheses.

A non-defining or non-essential clause gives us more information about the person or thing we are talking about. If a non-defining relative clause is removed from a sentence, we lose some detail, but the overall meaning of the sentence remains the same. Non-defining relative clauses are always set off from the rest of the sentence with commas or parentheses.