In English we use non-defining relative clauses to give extra information about a person, place or thing. Sentences with non-defining relative clauses have meaning with or without the clause.

Example sentences :

• This building, which was designed by Gaudi, is a tourist attraction in Barcelona.

This sentence also makes sense without the clause: This building is a tourist attraction in Barcelona.

• I went to Japan in 1984, when I was 14 years old, as an exchange student.

Notice the sentence makes perfect sense with or without the non-defining relative clause: I went to Japan in 1984 as an exchange student. 

Non-defining relative clauses must be put between commas, you can never leave out the relative pronoun, and you cannot use 'that' in place of 'who' or 'which'.

Example sentences :

• Alice Springs, where my mother grew up, is in the middle of Australia.
• My friend, who plays the trumpet, is in a concert next week.
• The sandals, which cost $99, come in black, brown or red.

Notes for use of non-defining relative clauses :

In non-defining relative clauses, 'who' or 'which' can also refer to a whole clause.

• The game of cricket lasts until every player is bowled out, which makes for a very long game! (which refers to the fact that the game of cricket lasts until every player is bowled out)

Non-defining relative clauses are more common in written English than spoken English, especially in more formal types of writing.