Public international law is 'The legal system governing the relationships between countries; more modernly, the law of international relations, embracing not only countries but also such participants as international organizations and individuals (such as those who invoke their human rights or commit war crimes)'.
Thomson Reuters Westlaw, Black's Law Dictionary
'Topics of international law include the law of the sea, the law of treaties, state jurisdiction, international criminal law, human rights, international humanitarian law, and the international law of the environment. Through international law an attempt is made to identify and advance common interests of humanity.'
Lexis Advance Pacific, Encyclopaedic Australian Legal Dictionary
Each jurisdiction's legal system will be structured differently - it is important to understand the source of primary law in a jurisdiction before you start your research.
Oxford University Press have created a history of international law, running from 1494 to the present.
This concise timeline maps the broad history of public international law with particular attention paid to the signing of major treaties, the foundation of fundamental institutions, the birth of major figures in international law and milestones in the development of some of the field’s best-known doctrines.