International law is the set of rules generally regarded and accepted as binding in relations between states and between nations. It serves as a framework for the practice of stable and organized international relations. International law differs from state-based legal systems in that it is primarily applicable to countries rather than to private citizens. National law may become international law when treaties delegate national jurisdiction to supranational tribunals such as the European Court of Human Rights or the International Criminal Court. Treaties such as the Geneva Conventions may require national law to conform to respective parts signed and ratified.

Professor Soares has provided a good answer to your question. But let me add that international law developed in large part through custom, which involves the actual practices of states combined with what indications they have given regarding whether they considered these actions - or refraining from other actions - to be legally obligatory. Secondly, international law is derived from agreements between states, with the intention either of creating new rules or of codifying existing rules. The answer is much more complicated than what I have said, but  this is a start.

In addition to the above excellent answers, international law is normally imposed voluntarily to nations that have agreed to be bound the any agreements or conventions as compared to national law, which is normally imposed on the citizens by the national legislative body.    

To All Contributors:

There are several competing as well as complimentary definitions of international law in the literature.

According to the narrowest of these definitions, international law is that legal order which regulates / governs the relations between independent (sovereign) states, a definition presented by the many contributors here and first established by the Permanent Court of International Justice in in the Lotus case of September 7, 1927 [Lotus 1927].

BEYOND SOVEREIGN STATES:

But that definition has been seen by International Law experts as excessively narrow, and a more nuanced appreciation has emerged as a new standard through the works of thinkers like Alfred Von Verdross (one of recognized founders ("Fathers") of International Law [Von Verdross 1949] [Kleinman 2012] [Simma 1995], Heinrich Brandweiner, among many others who followed. It is now recognized that the scope of International Law encompasses the legal order which governs relations between independent states and certain other legal but non-state communities such as:

1. Ecclesiastical Communities with international reach and consequence like the Catholic Church;

2. Recognized Armed Non-state Actors (armed NSAs) / belligerents like Al Qaeda, the Taliban, and ISIS, among others;

3. Global Collectivities like the European Union (an independent juridical entity separate from the component sovereign states within it); ditto for the Organization of American States (OAS);

4. Transnational / Intergovernmental Organizations like the United Nations;

among others with specialized status.

INDIVIDUALS AS SUBJECTS OF INTERNATIONAL LAW

5. But we have evolved to a far more inclusive definition of International Law that also recognize individuals themselves in certain narrowly proscribed cases as subjects of international law.

A. One example is that of International Military Tribunals (IMTs) such as those at Nuremberg and at Tokyo after WW II.

B. But also International Criminal Tribunals (ICT) such as the International Criminal Tribunal for the Former Yugoslavia (ICTY), the International Criminal Tribunal for Rwanda (ICTR), and the International Crimes Tribunal (Bangladesh), all these of which had an "umbra" of application inclusive of individuals under their juridical scope.

SUMMARY: THE SUBJECTS OF INTERNATIONAL LAW: THE NEW REGIME

A subject of International Law (as enumerated partially above) is an entity that possesses what is know as INTERNATIONAL LEGAL PERSONALITY, meaning one that is capable of possessing international rights and obligations, and having the capacity to take certain types of action on the international level.

So to capture the vibrant real-world operation of International Law in the current world, we must embrace these actors under the ambit of International Law - called the "Subjects" of International Law - and expand our definition of International Law:

THE NEW DEFINITION OF INTERNATIONAL LAW:

International Law is the legal order which regulates / governs the relations between:

1. Sovereign States

2. Ecclesiastical Communities that are global actors [Catholic Church . . . ]

3. Non-state Actors (armed NSAs) / belligerents [Al Qaeda, Taliban, ISIS . . . ]

3. Global Regulatory Collectivities [EU, OAS . . . ]

4. Transnational / Intergovernmental Non-Governmental Organizations (TNOs/IGOs/NGOs) [UN, IJC, ICC]

5. Individuals within the ambit of:

-- International Military Tribunals (IMTs) [Nuremberg, Tokyo]

-- International Criminal Tribunals (ICT) [ICTY, ICTR, ICT ((Bangladesh)]

And note that we have a rich body of International case law that serves to validate each of these delineated subjects as within the ambit of International Law[Besson & D'aspremont 2017] [Am. Soc’y Int’l L., “Sources and Evidence of International Law, in Benchbook on International Law. §I.B (Diane Marie Amann ed., 2014)].

SPECIAL SUBJECTS:

There are also controversial subjects about which there has emerged some uneasy consensus but with continuing controversy at the margins. These include;

6. Multinational Corporations (MNC)

7. Global Social Media Entities [Facebook, Twitter . . . ]

but significant debate continues as to the status of these entities and whether they constitute true formal subjects of International Law.

LESSONS LEARNED:

From the above, I have tried to imbue an appreciation of the nuances of International Law as a rich, living, vibrant discipline that continues to confront its own dynamics and its complex interactions and intersection with National Laws (when do the laws of Nations either trump, or are subordinate to, International Law) and with special "domains of application" such as:

- Public International Law (International Law when the scope is restricted solely to relationships between Nations themselves),

-- Private International Law (which deals strictly with controversies between private entities (people or corporations) which have a significant relationship to more than one nation (like the lawsuits arising from the toxic gas leak in Bhopal, India from Union Carbide industrial plants), and

-- International Humanitarian Law (a set of rules which seek, for humanitarian reasons, to limit the effects of armed conflict).

My thanks to Emanuela who launched this vital topic, and to Marcello, Glenn, Aminuddin, and Dhafer for their contributions.

SELECTIVE REFERENCES:

[Benchbook 2014] Am. Soc’y Int’l L., “Sources and Evidence of International Law, in Benchbook on International Law. §I.B (Diane Marie Amann ed., 2014).

[Besson & D'aspremont 2017] Samantha Besson and Jean D'aspremont (eds), The Oxford handbook on the sources of international law, Oxford : Oxford University Press, 2017, pp. 769-786

[Kleinman 2012] Kleinman T. Alfred Verdross as a Founding Father of International Constitutionalism? Goettingen J Internat Law 2012; 4(2): 385-416.

[Lotus 1927] S.S. Lotus (Fr. v. Turk.), 1927 P.C.I.J. (ser. A) No. 10 (Sept. 7). Publication: Publications of the Permanent Court of International Justice, Series A - No. 10; Collection of Judgments, A.W. Sijthoff’s Publishing Company, Leyden, 1927.

[Simma 1995] Simma B. The Contribution of Alfred Verdross to the Theory of International Law. Euro J Internat Law Jan 1995; 6(1):33–54.

[Von Verdross 1949] Von Verdross A. On the Concept of International Law. Am J Internat Law Jul 1949; 43(3):435-440.

هو مجموعة من القواعد القانونية التي تهدف تنظيم العلاقات التي تنشأ بين الكيانات الدولية والتي قد يكون مصدرها الاتفاقيات الدولية أو العرف الدولي في أوقات السلم والحرب

I must say it is an easy but challenging question to answer. Professor Soares already gave an excellent comprehensive answer, and his response reminds me of the definition given by Professor Oppenheim. Please let me add a bit, that body of rules or principles which regulates the relationship of the States with each other, as well as, their relationship and partnership with other international actors.