"Est-ce nécessaire ?"
Translation:Is it necessary?
I still don't feel comfortable enough yet to explain where you'd use one or the other, but if you'd use "c'est," then inverting it gives you "est-ce," and if you'd use "il est" then you'd use "est-il." You've surely seen it before, at least in questions beginning with "est-ce que."
The difference is explained very well here: