Foreign policy dictates how a country will act with respect to other countries politcally, socially, economically, and militarily, and to a somewhat lesser extent, how it behaves towards non-state actors. Foreign policy can also be known as international relations policy or simply diplomacy. It seems likely that foreign policy, in some form, has been around since the early days of humanity on the plains of Africa, when large tribes would presumably interact from time to time without engaging in all-out war. Today, foreign policy is handled by foreign ministers, ambassadors, and/or the Secretary of State (in the US).
Although foreign policy has always been important, it is especially true today, when air travel makes the world smaller and more interconnected, and powerful weapons make the risk of diplomatic breakdown all the more dire. Many thinkers in the field put the genesis of modern foreign policy and statesmanship with Cardinal Richelieu, a statesman in early 17th century France. Richelieu became famous for consolidating French power, making France among the first of the Great Powers, and feuding with the Hapsburg dynasty, which ruled in both Austria and Spain. He ordered all the castles of the lesser nobility and feudal lords to be razed, causing the royalty of France to become more powerful than ever before.
The 400 years since Richelieu has been marked by intense foreign policy interactions and negotiations among the world's most powerful nations, especially those in Europe, where the world's power was centralized until the mid 20th century, when nations like America and China came into their own. An important milestone in foreign policy and international relations came in 1919, when World War I came to a close with the Treaty of Versailles (which would anger the Germans into starting WWII), and the League of Nations was established. The League of Nations was the first true international organization of countries, and was a forerunner of the modern-day United Nations.
Today, many official proclamations and foreign policy actions are channeled through the United Nations, which serves more like an international forum rather than any unified political body. There are many thinkers in foreign policy that either support the United Nations, calling it a step to a more unified and civilized world, or repudiate it, calling it an useless charade. The most important body in the United Nations is the Security Council, a board consisting of the countries of the United States, United Kingdom, Russia, China, and France. This makeup has been criticized by other countries as merely representing the winners of WWII, and not really reflective of the international will. Still, the UN Security Council does get considerable attention and press, and while its influence over world events is not unlimited, it surely is strong.