International law is a loose network of treaty obligations and judicial precedents from around the world. It is very vague and constantly changing. Some of it is set in international organizations that have some sort of enforcement power due to treaties or due to dependence on those organizations by nations and other vital services, such as the IMF, World Bank, UN, etc. Other international law is set at large conferences in which many nations sign on to a single treaty, such as the Geneva Convention that set the laws of war and also allows for war crimes trials and precedents to be set in the Hague.
Other international law is determined by Courts that apply it all over the world when determining international disputes and look to the law of their own nation to figure out what to do, but use the rulings of international Courts to the extent they are useful when their own national law is vague and other Courts in other countries have grappled with similar issues already. Many international contracts specify that they will use the law of the State of New York to apply to contract disputes and also often international parties consent to arbitration, allowing the private rules of the arbitration entity to determine what happens (those rules are often based on the law in a particular jurisdiction, but they don't have to be).