Sources of International Law: According to the Article 38 of the Statute of ICJ

Article 38 of the Statute of the International Court of Justice (ICJ) is a complete statement of the sources of International Law.

Article 38(1) (a); International Conventions and Treaties, whether general or particular, establishing rules expressly recognized by the contesting states.
Article 38(1) (b); International Custom, as evidence of a general practice accepted by state as law.
Article 38(1) (c); the general principles of law recognized by civilized nations.
Article 38(1) (d); Judicial Decisions and teachings of the most highly qualified publicists of the different nations.
International Conventions and Treaties:

According to Vienna Convention on Law of treaties,1969Article 2: Treaty is a legal agreement concluded between two or more states in written form and governed by International Law. Treaties are sometimes referred to convention, protocol, accord etc. Treaties, or International Conventions, can be bilateral (between two nations) or multilateral (between many nations). Examples are Nuclear Non-Proliferation Treaty, Sino- Pakistan free trade agreement, International Convention for the safety of life at sea, Treaty on the accession of Crimea to Russia, Minsk2 protocol etc. Treaty between two or more States prescribing rules and regulation binding between the parties only is called Treaty Contracts. It is indirect source but may lead to its formation. The rules and procedures governing UN’s procedural matters i.e. its budget, finances etc. Treaties that lays down general principles of law and binding is called law making treaties. It is a direct source of International Law i.e Treaty on diplomatic immunity.

Customs: It is the original and the oldest source of international law.

Essentials of customs are;

Long Duration
Constant International Practice
Opinion Juris: an action is carried as a legal obligation
Custom has opinion juiris means the custom is followed as a legal obligation. Usage, on the other hand, lacks opinion juris. For instance, giving a red carpet reception to foreign head of the States is a usage but giving immunity to the ambassadors has always remained an international custom.

General Principles of Law:

It is recognized by civilized nations in its decision making. It is the principle of natural justice in which both the parties should be heard, nobody can be a judge in his own case. Chorzow Factory case is its example. General Principles are most important source of law when no treaty or CIL has addressed an issue.

Judicial Decisions and Writing of Jurists:

Every year 1000 books and 3000 articles are written on IL (Akehurst’s Modern Introduction to International law). The writings of qualified jurists like Grotius, Vattel etc have been quoted by the ICJ and other International tribunals. Judicial decisions are the subsidiary means of International Law. Under the head Judicial Decisions, contribution of ICJ, ICC and others international tribunals are significant.