Differentiate the terms jus in bello and jus ad bellum

Define and differentiate the terms jus in bello and jus ad bellum, then use Michael Walzer's article (or speech) on "Terrorism and Just War", to answer the following question: Under what circumstances, if any, is it permissible to kill (a) combatants and (b) noncombatants in war? Give historical examples of both situations to illustrate your point.