As defined in in Article 38(1) of the Statute of the International Court of Justice (“ICJ”) legal sources of international law, include
1) treaties/conventions between countries
2) state practice and opinio juris (as elements of customary law),
3) general principles of law, and
4) judicial decisions and scholarly work.
Customary international law,derives from "a general practice accepted as law". Such practice can be found in official accounts of military operations but is also reflected in a variety of other official documents, including military manuals, national legislation and case law. The requirement that this practice be "accepted as law" is often referred to as "opinio juris". This characteristic sets practices required by law apart from practices followed as a matter of policy.
Treaties, such as the four Geneva Conventions of 1949, are written conventions in which States formally establish certain rules. Treaties bind only those States which have expressed their consent to be bound by them, usually through ratification. In 1969 the Vienna Convention on the Law of Treaties was adopted. According to Article 2 of this agreement, a 'treaty’ means 'an international agreement concluded between States in written form and governed by international law, whether embodied in a single instrument or in two or more related instruments and whatever its particular designation.'
Treaties are typically characterized by two main categories: multilateral and bilateral.
When researching treaties, first determine if you are working with a bilateral or multilateral treaty, you can then identify the primary treaty document using a citation, or by searching a depository, database, or compilation by category, subject, or keyword.
Treaty terminology: When researching treaties, note there are terms that have specialized meanings within this area of research. For example, "ratification" of a treaty occurs when the parties consent to be bound by the agreement, whereas, an "accession" to a treaty occurs when a party consents to be bound by a previously ratified agreement. You may also see the term "reservation" while researching; a reservation is an act by a party where they exclude a certain provision of a treaty, but agree to the rest of the provisions. The UN has a Glossary of terms related to Treaty actions. When researching, awareness of these terms and their specific meanings will enable you to identify treaties, related instruments, and acts more effectively.