International law is the set of rules that are accepted as binding by states in their relations with each other and individuals. It emerged in the 16th century from thinkers like Grotius and serves as the framework for organized international relations. There are various sources of international law, including treaties, customary practice, and general legal principles. The key subjects that international law applies to are states and non-state actors like individuals and international organizations. The 10 main principles of international law include sovereign equality of states, non-use of force, territorial integrity, and human rights.