Foreign policy is the sum of a state's objectives, strategies and instruments for conducting its relations with external actors in pursuit of its national interest — the bridge between a state's internal purposes and the external environment. It is the continuity of choices: whom to befriend, what to trade, which treaties to sign, where to align and where to abstain.
National interest is foreign policy's compass: the enduring needs of the state — survival(sovereignty, territorial integrity, political independence), security, economic wellbeingand prestige/dignity. Morgenthau's canonical elements: sovereignty, territorial integrity, political independence, national honour. Its layers: core/vital interests (defend at all costs), major interests, peripheral interests. The concept's critics warn it can rationalize anything ("interest defined as power" as the last argument of every establishment); its defenders answer that without a compass there is only drift.
Foreign policy's instruments (the toolkit): diplomacy (Unit 5), economic statecraft (trade, aid, sanctions, investment), military instruments (alliance, deterrence, peacekeeping), law (treaties, arbitration — law as the small state's shield), and public/cultural diplomacy (soft power, Unit 1).