There are two main divisions of International Law—the law of Peace or the rules governing the relations of states in times of peace and the law of war, the rules governing the relations of state during war. The first category of law also contains rules governing the position and rights of neutrals and their relations with the belligerent states.
International law is as old as hills. Even in ancient times, there were certain rules according to which the relations among the tribal states were regulated.
For instance, in ancient India, there existed a regular set of rules according to which ambassadors were accredited, wars were declared and conducted, diplomatic relations established and broken, and treaties and alliances negotiated and concluded.
It is a well-known fact that the Great War of Mahabharata was fought according to certain set principles. As for example, Fighting used to be stopped at sun set and resumed in the morning. At night, people belonging to hostile warring camps could meet and exchange friendly greetings.
In Europe International law was developed by Romans under the name of Jus Centium or law of nations. However, the credit goes to Dutch Jurist Hugo Grotious for writing a treatise on International law—law of war and law of peace. Modern International law has developed out of these two principles.
Principles of International law were further developed by jurists like Wolf, Valtal, Hall, Lawrence and Oppenheim. International law as such is derived from various sources which are discussed at the end of this question.