The information provided in a defining relative clause is crucial in understanding the meaning of the sentence.
Example: The woman who lives in apartment No. 34 has been arrested.
The document that I need has 'important' written at the top.
The purpose of a defining relative clause is to clearly define who or what we are talking about. Without this information, it would be difficult to know who or what is meant.
Example: The house is being renovated.
In this case it is not necessarily clear which house is being renovated.
Non-defining Relative Clauses
Non-defining relative clauses provide interesting additional information which is not essential to understanding the meaning of the sentence.
Example: Mrs. Jackson, who is very intelligent, lives on the corner.
Correct punctuation is essential in non-defining relative clauses. If the non-defining relative clause occurs in the middle of a sentence, a comma is put before the relative pronoun and at the end of the clause. If the non-defining relative clause occurs at the end of a sentence, a comma is put before the relative pronoun.
NOTE: In defining relative clauses there are no commas.Using Relative Clauses