Well, there are two types of relative clauses: defining and non-defining. Defining relative clauses tell us which person or thing ( or what kind of person or thing) the speaker means, while non-defining relative clauses give us extra information about the person or thing and in these clauses we use commas (,). 1) Let us use your example: The woman who lives next door works in a bank. ('who lives next door' is defining relative clause, if we wrote: The woman works in a bank. without this relative clause, then we wouldn't know which woman we are talking about,, but if we say The woman who lives next door works in a bank. then we know that the woman who works in the bank is the woman who lives next door, so we know who she is.) 2) My brother Jim, who lives in London, is a doctor. ('who lives in London' is non-defining relative clause, because we already know who we are speaking about, it just gives us extra information. If we wrote the sentence without this clause, it would be: 'My brother Jim is a doctor.' and we could say just that, but if we add 'who lives in London' then we know something more about him. If you can rewrite the sentence without relative clause, then it is non-defining relative clause, but if your sentence does not make sense without relative clause and you do not know who you are talking about then it is defining relative clause. Relative pronouns are: who, which, whose, that... We use who only for people, which for animals and things, and we can use that for both people and things. If there is anything else you don't understand, just ask