THE FIRST QUESTION TO ANSWER IS: WHAT IS A TREATY?
Treaty is a generic term embracing all instruments binding under international law, regardless of their formal designation, concluded between two or more international juridical persons.
Thus, treaties may be concluded between: States; International organizations with treaty-making capacity and States; or International organizations with treaty-making capacity.
The application of the term treaty, in the generic sense, signifies that the parties intend to create rights and obligations enforceable under international law.
The Vienna Convention 1969 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" (article 2(1)(a)).
Accordingly, conventions, agreements, protocols, and exchange of letters or notes may all constitute treaties.
A treaty must be governed by international law and is normally in written form. Although the Vienna Convention 1969 does not apply to non-written agreements, its definition of a treaty states that the absence of writing does not affect the legal force of international agreements.
No international rules exist as to when an international instrument should be entitled a treaty. However, usually the term treaty is employed for instruments of some gravity and solemnity.
See article 2(1)(a) of the Vienna Convention 1969. See generally Vienna Convention 1969 and Vienna Convention 1986. Bilateral treaty A bilateral treaty is a treaty between two parties. Multilateral treaty A multilateral treaty is a treaty between more than two parties.
Source: United Nations Treaty Handbook
ABOUT TREATY LAW AND THE LAW OF TREATIES
There is a slight (albeit rarely understood) difference between Treaty Law and the Law of Treaties.
Treaty Law deals with the subject matter of treaties, i.e. how treaties addresses particular issues such as the Law of the Sea or extradition measures.
The Law of Treaties deals with how treaties come into being and how to process of making and administering treaties is regulated.
This site, treatylaw.org deals with both topics.