Restrictive or Non-restrictive?
Restrictive relative clauses are sometimes called defining relative clauses, or identifying relative clauses. Similarly, non-restrictive relative clauses are called non-defining or non-identifying relative clauses.
In English a non-restrictive relative clause is preceded by a pause in speech or a comma in writing, unlike a restrictive clause.
For example:-
- See more at: http://www.learnenglish.de/grammar/clausetext.html#sthash.pYoh7pBq.dpuf