used to add extra information to a previous clause, in writing usually after a comma: That bar on Milton Street, which by the way is very nice, is owned by Trevor's brother. She says it's Charlotte's fault, which is rubbish, and that she blames her. Anyway, that evening, which I'll tell you more about later, I ended up staying at Rachel's place. It's the third in a sequence of three books, the first of which I really enjoyed. He showed me round the town, which was very kind of him.
used as the subject or object of a verb to show what thing or things you are referring to, or to add information about the thing just mentioned. It is usually used for things, not people: These are principles which we all believe in. You know that little Italian restaurant - the one which I mentioned in my letter? Is that the film in which he kills his mother? The death of his son was an experience from which he never fully recovered. It isn't a subject to which I devote a great deal of thought.
used to introduce a clause which reports something or gives further information, although it can often be omitted: She said (that) she'd collect it for me after work. Is it true (that) she's gone back to teaching? We'll be there at about 7.30, provided/providing (that) there's a suitable train. It was so dark (that) I couldn't see anything.