a non-restrictive RELATIVE CLAUSE gives additional information about a particular person or thing rather than saying which person or thing is being mentioned. For example, in the sentence 'Perry, who is 22, was arrested yesterday', the phrase 'who is 22' is a non-restrictive clause.
technical a part of a sentence that says which particular person or thing you are talking about. For example in 'the man who came to dinner', the phrase 'who came to dinner' is a restrictive clause