The use of the expression is (of course) when you get the feeling that you have seen something before, when you know full well that you haven't. Other tricks that the brain plays are "jamais vu" - the opposite of deja ve - when you react to something familiar as if you had never seen it before, and "pesque vu" - almost seen, when you get the feeling that you almost saw something out of the cornere of your eye, but when you look directly, it's not there. (Not really sure about the last one, but that's what I think it means.)