A binding agreement under international law concluded by subjects of international law, namely states and international organizations. Treaties can be called by many names: treaties, international agreements, protocols, covenants, conventions, exchanges of letters, exchanges of notes, etc.; however all these are equally treaties, and the rules are the same irregardless of what the treaty is called.
Most of the rules regarding treaties are contained in the Vienna Convention on the Law of Treaties between States, and the Vienna Convention on the Law of Treaties between States and International Organizations or between International Organizations. Most states have ratified these treaties. There is also the Vienna Convention on Succession of States with Respect to Treaties, however it has not entered into force, and many states reject its provisions as not adequately reflecting the customary international law on the subject.
Treaties are divided into two types: bilateral and multilateral. A multilateral treaty has several parties, and establishes rights and obligations between each party and every other party. Multilateral treaties are often, but not always, open to any state.
Bilateral states by contrast are negotiated between a limited number of states, most commonly only two, establishing legal rights and obligations between those two states only. It is possible however for a bilateral treaty to have more than two parties; consider for instance the bilateral treaties between Switzerland and the EU following the Swiss rejection of the EEA agreeement. Each of these treaties has seventeen parties. These however are still bilateral, not multilateral, treaties. The parties are divided into two groups, the Swiss ("on the one part") and the European Community and its member states ("on the other part"). The treaty establishes rights and obligations between the Swiss and the EC and the member states severally; it does not establish any rights and obligations amongst the EC and its member states.
Define these terms: Adoption, signature, ratification, reservation, declaration, accession, acceptance, approval, withdrawal, denunciation.
United States law
In United States law, there is a distinction between 'treaties' and 'international agreements': ratification of treaties requires the advice and consent of the Senate, while 'international agreements' can be ratified by the joint Executive-Congressional action or by Executive action alone. This distinction applies only within US law; no such distinction exists in international law, and treaties and international agreements concluded by the United States are equally treaties within the meaning of international law.