This document provides an introduction to international law. It defines international law as the set of rules that countries follow in dealing with each other. It notes there are three main categories: public international law regarding relations between sovereign states; private international law addressing jurisdiction in legal cases with international aspects; and supranational law involving collective laws agreed upon by states. The document outlines some key characteristics of international law, such as its decentralized nature in lawmaking and enforcement. It also discusses theories for why international law is considered binding on states and other actors. Finally, it briefly summarizes some functions and weaknesses of international law.