What are Treaties & International Agreements?
The Vienna Convention on the Law of Treaties (.pdf) defines a treaty as "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 can be referred to by a number of different names: international conventions, international agreements, covenants, final acts, charters, memorandums of understandings (MOUs), protocols, pacts, accords, and constitutions for international organizations. Usually these different names have no legal significance in international law (see next section for the difference in U.S. law). Treaties may be bilateral (two parties) or multilateral (between several parties) and a treaty is usually only binding on the parties to the agreement. An agreement "enters into force" when the terms for entry into force as specified in the agreement are met. Bilateral treaties usually enter into force when both parties agree to be bound as of a certain date.
For definitions of key terms, see the U.N. Treaty Reference Guide.
For more information, see Thomas Buergenthal, Public International Law in a Nutshell or the Encyclopedia of Public International Law, vol. 7, pps. 459-514.