A-defining relative clause tells which noun we are talking about:I like the woman who lives next door.(If I don't say 'who lives next door', then we don't know which woman I mean).B- non-defining relative clause gives us extra information about something. We don't need this information to understand the sentence.I live in London, which has some fantastic parks.(Everybody knows where London is, so 'which has some fantastic parks' is extra information).