There are two kinds of adjective clauses in English: defining and non-defining. This lesson explains non-defining adjective clauses.
What's a non-defining adjective clause?
First, some examples:
|Canada, which is located north of the USA, has a long winter.||which is located north of the USA||The clause describes “Canada”. It gives us extra information about Canada.|
|William Shakespeare, who lived over four hundred years ago, wrote the most famous plays in the English language.||who lived over four hundred years ago||The clause describes “William Shakespeare”, and gives us extra information about him.|
Please notice how the non-defining adjective clause gives extra information about the noun it describes. It doesn't tell us which one we're talking about — in the examples above, everybody knows “Canada” and “William Shakespeare”, so it doesn't define them.